Return early on GetProviderSchema RPC responses with error diagnostics

[bflad]

Closes #31047

Prevent potential panics and immediately return provider-defined errors diagnostics.

Previously in internal/plugin6:

--- FAIL: TestGRPCProvider_GetSchema_ResponseErrorDiagnostic (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x70 pc=0x17fa752]

goroutine 13 [running]:
testing.tRunner.func1.2({0x191a100, 0x2236330})
	/usr/local/Cellar/go/1.18.2/libexec/src/testing/testing.go:1389 +0x24e
testing.tRunner.func1()
	/usr/local/Cellar/go/1.18.2/libexec/src/testing/testing.go:1392 +0x39f
panic({0x191a100, 0x2236330})
	/usr/local/Cellar/go/1.18.2/libexec/src/runtime/panic.go:838 +0x207
github.com/hashicorp/terraform/internal/plugin6/convert.ProtoToConfigSchema(0x0)
	/Users/bflad/src/github.com/hashicorp/terraform/internal/plugin6/convert/schema.go:110 +0x52
github.com/hashicorp/terraform/internal/plugin6/convert.ProtoToProviderSchema(...)
	/Users/bflad/src/github.com/hashicorp/terraform/internal/plugin6/convert/schema.go:98
github.com/hashicorp/terraform/internal/plugin6.(*GRPCProvider).GetProviderSchema(0xc00004a200)
	/Users/bflad/src/github.com/hashicorp/terraform/internal/plugin6/grpc_provider.go:152 +0x29a
github.com/hashicorp/terraform/internal/plugin6.TestGRPCProvider_GetSchema_ResponseErrorDiagnostic(0x0?)
	/Users/bflad/src/github.com/hashicorp/terraform/internal/plugin6/grpc_provider_test.go:158 +0x265
testing.tRunner(0xc0001031e0, 0x1a733d8)
	/usr/local/Cellar/go/1.18.2/libexec/src/testing/testing.go:1439 +0x102
created by testing.(*T).Run
	/usr/local/Cellar/go/1.18.2/libexec/src/testing/testing.go:1486 +0x35f

Previously in internal/plugin:

--- FAIL: TestGRPCProvider_GetSchema_ResponseErrorDiagnostic (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x70 pc=0x18a2732]

goroutine 7 [running]:
testing.tRunner.func1.2({0x1a5e720, 0x250be50})
	/usr/local/Cellar/go/1.18.2/libexec/src/testing/testing.go:1389 +0x24e
testing.tRunner.func1()
	/usr/local/Cellar/go/1.18.2/libexec/src/testing/testing.go:1392 +0x39f
panic({0x1a5e720, 0x250be50})
	/usr/local/Cellar/go/1.18.2/libexec/src/runtime/panic.go:838 +0x207
github.com/hashicorp/terraform/internal/plugin/convert.ProtoToConfigSchema(0x0)
	/Users/bflad/src/github.com/hashicorp/terraform/internal/plugin/convert/schema.go:104 +0x52
github.com/hashicorp/terraform/internal/plugin/convert.ProtoToProviderSchema(...)
	/Users/bflad/src/github.com/hashicorp/terraform/internal/plugin/convert/schema.go:92
github.com/hashicorp/terraform/internal/plugin.(*GRPCProvider).GetProviderSchema(0xc00004a600)
	/Users/bflad/src/github.com/hashicorp/terraform/internal/plugin/grpc_provider.go:149 +0x29a
github.com/hashicorp/terraform/internal/plugin.TestGRPCProvider_GetSchema_ResponseErrorDiagnostic(0x0?)
	/Users/bflad/src/github.com/hashicorp/terraform/internal/plugin/grpc_provider_test.go:130 +0x265
testing.tRunner(0xc0001031e0, 0x1be9500)
	/usr/local/Cellar/go/1.18.2/libexec/src/testing/testing.go:1439 +0x102
created by testing.(*T).Run
	/usr/local/Cellar/go/1.18.2/libexec/src/testing/testing.go:1486 +0x35f

[jbardin]

Thanks for the additional tests!

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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