fix(ls): panic when failing to load root config.

[i4ki]

What this PR does / why we need it:

When an error happens while looking for the root config, the language server panics with the stack trace below:

2024-02-10T14:38:52Z DBG handling request. action=server.Handler() method=textDocument/didOpen params={"textDocument":{"languageId":"terramate","text":"terramate {\n    config {\n        git {\n            default_branch = \"trunk\"\n        }\n    }\n}\n","uri":"file:///Users/tiagodemoura/src/vscode-terramate/testFixture/invalid/multiple-tm-config-git/tm2.tm","version":1}} workspace=/Users/tiagodemoura/src/vscode-terramate
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x1012a3d04]

goroutine 8 [running]:
github.com/terramate-io/terramate/config.(*Root).Tree(...)
	/Users/tiagodemoura/src/terramate/config/config.go:125
github.com/terramate-io/terramate/ls.(*Server).checkFiles(0x140001102d0, {0x140002ce380, 0x2, 0x101287df4?}, {0x14000024697, 0x5a}, {0x1400001ab40, 0x5e})
	/Users/tiagodemoura/src/terramate/ls/ls.go:370 +0x64
github.com/terramate-io/terramate/ls.(*Server).checkAndReply(0x14000024690?, {0x101414268, 0x140001d1040}, 0x140002ce320, {0x14000024697, 0x5a}, {0x1400001ab40, 0x5e})
	/Users/tiagodemoura/src/terramate/ls/ls.go:324 +0x10c
github.com/terramate-io/terramate/ls.(*Server).handleDocumentOpen(0x73?, {0x101414268, 0x140001d1040}, 0x1400014cab8?, {0x1264f6e98, 0x140002c2e70}, {{0x101412b48, 0x14000027ff0}, 0xff, {0x0, ...}, ...})
	/Users/tiagodemoura/src/terramate/ls/ls.go:184 +0xf4
github.com/terramate-io/terramate/ls.(*Server).Handler(0x140001102d0, {0x101414268, 0x140001d1040}, 0x140002ce320, {0x1264f6e98?, 0x140002c2e70?})
	/Users/tiagodemoura/src/terramate/ls/ls.go:90 +0x418
go.lsp.dev/jsonrpc2.(*conn).run(0x140001d2ff0, {0x101414268, 0x140001d1040}, 0x140001e81f0)
	/Users/tiagodemoura/go/pkg/mod/go.lsp.dev/jsonrpc2@v0.10.0/conn.go:206 +0x258
created by go.lsp.dev/jsonrpc2.(*conn).Go in goroutine 1
	/Users/tiagodemoura/go/pkg/mod/go.lsp.dev/jsonrpc2@v0.10.0/conn.go:189 +0xa8
[Error - 14:38:52] Connection to server got closed. Server will not be restarted.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

yes, fixes a LS bug.

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[github-actions]

metric: time/op
CloudReadLines-4: old 883µs ± 4%: new 879µs ± 4%: delta: 0.00%
CloudReadLine-4: old 6.67ms ± 3%: new 6.56ms ± 2%: delta: -1.77%
ListFiles-4: old 49.8µs ± 2%: new 50.0µs ± 2%: delta: 0.00%
Generate-4: old 2.44s ± 3%: new 2.48s ± 2%: delta: 1.38%
GenerateRegex-4: old 1.73s ± 2%: new 1.74s ± 1%: delta: 0.78%
TokensForExpressionComplex-4: old 1.11ms ± 1%: new 1.12ms ± 0%: delta: 0.92%
TokensForExpressionPlainStringNoNewline-4: old 912ns ± 3%: new 933ns ± 1%: delta: 2.29%
TokensForExpressionStringWith100Newlines-4: old 33.2µs ± 2%: new 33.5µs ± 3%: delta: 0.00%
TokensForExpressionObjectWith100KeysWithNumberValues-4: old 1.24ms ± 3%: new 1.24ms ± 3%: delta: 0.00%
TokensForExpression-4: old 1.08ms ± 3%: new 1.09ms ± 3%: delta: 1.06%
PartialEvalComplex-4: old 417µs ± 4%: new 421µs ± 2%: delta: 0.82%
PartialEvalSmallString-4: old 3.49µs ± 2%: new 3.51µs ± 2%: delta: 0.00%
PartialEvalHugeString-4: old 1.82ms ± 3%: new 1.81ms ± 2%: delta: 0.00%
PartialEvalHugeInterpolatedString-4: old 4.62ms ± 1%: new 4.65ms ± 2%: delta: 0.00%
PartialEvalObject-4: old 21.1µs ± 2%: new 20.8µs ± 2%: delta: -1.62%
TmAllTrueLiteralList-4: old 5.42ms ± 3%: new 5.34ms ± 1%: delta: -1.46%
TmAllTrueFuncall-4: old 139µs ± 3%: new 138µs ± 2%: delta: -1.04%
TmAnyTrueLiteralList-4: old 126ms ± 3%: new 125ms ± 1%: delta: -1.42%
TmAnyTrueFuncall-4: old 139µs ± 2%: new 138µs ± 1%: delta: 0.00%
TmTernary-4: old 2.50µs ± 2%: new 2.50µs ± 2%: delta: 0.00%
TmTry-4: old 50.6µs ± 1%: new 50.6µs ± 2%: delta: 0.00%
metric: alloc/op
CloudReadLines-4: old 3.12MB ± 0%: new 3.12MB ± 0%: delta: 0.00%
CloudReadLine-4: old 3.37MB ± 0%: new 3.37MB ± 0%: delta: 0.00%
ListFiles-4: old 22.0kB ± 0%: new 22.0kB ± 0%: delta: 0.00%
Generate-4: old 2.31GB ± 0%: new 2.31GB ± 0%: delta: 0.00%
GenerateRegex-4: old 950MB ± 0%: new 949MB ± 0%: delta: 0.00%
TokensForExpressionComplex-4: old 412kB ± 0%: new 412kB ± 0%: delta: 0.00%
TokensForExpressionPlainStringNoNewline-4: old 592B ± 0%: new 592B ± 0%: delta: 0.00%
TokensForExpressionStringWith100Newlines-4: old 14.0kB ± 0%: new 14.0kB ± 0%: delta: 0.00%
TokensForExpressionObjectWith100KeysWithNumberValues-4: old 402kB ± 0%: new 402kB ± 0%: delta: 0.00%
TokensForExpression-4: old 412kB ± 0%: new 412kB ± 0%: delta: 0.00%
PartialEvalComplex-4: old 353kB ± 0%: new 353kB ± 0%: delta: 0.00%
PartialEvalSmallString-4: old 1.74kB ± 0%: new 1.74kB ± 0%: delta: 0.00%
PartialEvalHugeString-4: old 166kB ± 0%: new 166kB ± 0%: delta: 0.00%
PartialEvalHugeInterpolatedString-4: old 4.38MB ± 0%: new 4.38MB ± 0%: delta: 0.00%
PartialEvalObject-4: old 20.4kB ± 0%: new 20.4kB ± 0%: delta: 0.00%
TmAllTrueLiteralList-4: old 1.74MB ± 0%: new 1.74MB ± 0%: delta: 0.00%
TmAllTrueFuncall-4: old 45.5kB ± 0%: new 45.5kB ± 0%: delta: 0.00%
TmAnyTrueLiteralList-4: old 37.9MB ± 0%: new 37.9MB ± 0%: delta: 0.00%
TmAnyTrueFuncall-4: old 45.6kB ± 0%: new 45.6kB ± 0%: delta: 0.00%
TmTernary-4: old 1.11kB ± 0%: new 1.11kB ± 0%: delta: 0.00%
TmTry-4: old 11.1kB ± 0%: new 11.1kB ± 0%: delta: 0.00%
metric: allocs/op
CloudReadLines-4: old 5.54k ± 0%: new 5.54k ± 0%: delta: 0.00%
CloudReadLine-4: old 60.0k ± 0%: new 60.0k ± 0%: delta: 0.00%
ListFiles-4: old 321 ± 0%: new 321 ± 0%: delta: 0.00%
Generate-4: old 25.9M ± 0%: new 25.9M ± 0%: delta: 0.00%
GenerateRegex-4: old 18.3M ± 0%: new 18.3M ± 0%: delta: 0.00%
TokensForExpressionComplex-4: old 4.93k ± 0%: new 4.93k ± 0%: delta: 0.00%
TokensForExpressionPlainStringNoNewline-4: old 21.0 ± 0%: new 21.0 ± 0%: delta: 0.00%
TokensForExpressionStringWith100Newlines-4: old 328 ± 0%: new 328 ± 0%: delta: 0.00%
TokensForExpressionObjectWith100KeysWithNumberValues-4: old 3.29k ± 0%: new 3.29k ± 0%: delta: 0.00%
TokensForExpression-4: old 4.93k ± 0%: new 4.93k ± 0%: delta: 0.00%
PartialEvalComplex-4: old 2.83k ± 0%: new 2.83k ± 0%: delta: 0.00%
PartialEvalSmallString-4: old 23.0 ± 0%: new 23.0 ± 0%: delta: 0.00%
PartialEvalHugeString-4: old 35.0 ± 0%: new 35.0 ± 0%: delta: 0.00%
PartialEvalHugeInterpolatedString-4: old 23.1k ± 0%: new 23.1k ± 0%: delta: 0.00%
PartialEvalObject-4: old 125 ± 0%: new 125 ± 0%: delta: 0.00%
TmAllTrueLiteralList-4: old 13.6k ± 0%: new 13.6k ± 0%: delta: 0.00%
TmAllTrueFuncall-4: old 460 ± 0%: new 460 ± 0%: delta: 0.00%
TmAnyTrueLiteralList-4: old 252k ± 0%: new 252k ± 0%: delta: 0.00%
TmAnyTrueFuncall-4: old 462 ± 0%: new 462 ± 0%: delta: 0.00%
TmTernary-4: old 26.0 ± 0%: new 26.0 ± 0%: delta: 0.00%
TmTry-4: old 144 ± 0%: new 144 ± 0%: delta: 0.00%