Fix panic calling compile with wrong FQBN

[silvanocerza]

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• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• [x Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Fixes a panic on compile.

• What is the current behavior?

$ arduino-cli compile -b aduino:avr:uno ~/Documents/Arduino/Blink/
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x38 pc=0x10128f424]

goroutine 1 [running]:
github.com/arduino/arduino-cli/arduino/cores.(*Platform).String(...)
	/Users/alien/workspace/arduino-cli/arduino/cores/cores.go:262
github.com/arduino/arduino-cli/commands/compile.Compile(0x1016a14a0, 0x1400003c0c8, 0x140001c5c58, 0x10168e3a8, 0x14000010018, 0x10168e3a8, 0x14000010020, 0x14000412d00, 0x0, 0x0, ...)
	/Users/alien/workspace/arduino-cli/commands/compile/compile.go:127 +0xa44
github.com/arduino/arduino-cli/cli/compile.run(0x140003faf00, 0x140004124e0, 0x1, 0x3)
	/Users/alien/workspace/arduino-cli/cli/compile/compile.go:194 +0x41c
github.com/spf13/cobra.(*Command).execute(0x140003faf00, 0x140004124b0, 0x3, 0x3, 0x140003faf00, 0x140004124b0)
	/Users/alien/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:860 +0x1d0
github.com/spf13/cobra.(*Command).ExecuteC(0x14000301680, 0x1, 0x1, 0x0)
	/Users/alien/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:974 +0x264
github.com/spf13/cobra.(*Command).Execute(...)
	/Users/alien/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:902
main.main()
	/Users/alien/workspace/arduino-cli/main.go:31 +0xd4

$ arduino-cli compile -b arduino:av:uno ~/Documents/Arduino/Blink/
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x38 pc=0x1032df424]

goroutine 1 [running]:
github.com/arduino/arduino-cli/arduino/cores.(*Platform).String(...)
	/Users/alien/workspace/arduino-cli/arduino/cores/cores.go:262
github.com/arduino/arduino-cli/commands/compile.Compile(0x1036f14a0, 0x140001a4008, 0x1400009bc58, 0x1036de3a8, 0x140001b0008, 0x1036de3a8, 0x140001b0010, 0x1400043cd00, 0x0, 0x0, ...)
	/Users/alien/workspace/arduino-cli/commands/compile/compile.go:127 +0xa44
github.com/arduino/arduino-cli/cli/compile.run(0x14000426f00, 0x1400043c4e0, 0x1, 0x3)
	/Users/alien/workspace/arduino-cli/cli/compile/compile.go:194 +0x41c
github.com/spf13/cobra.(*Command).execute(0x14000426f00, 0x1400043c4b0, 0x3, 0x3, 0x14000426f00, 0x1400043c4b0)
	/Users/alien/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:860 +0x1d0
github.com/spf13/cobra.(*Command).ExecuteC(0x1400032d680, 0x1, 0x1, 0x0)
	/Users/alien/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:974 +0x264
github.com/spf13/cobra.(*Command).Execute(...)
	/Users/alien/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:902
main.main()
	/Users/alien/workspace/arduino-cli/main.go:31 +0xd4

• What is the new behavior?

$ arduino-cli compile -b aduino:avr:uno ~/Documents/Arduino/Blink/

Error during build: Platform 'arduino:vr' not found: platform not installed                                                                        

$ arduino-cli compile -b arduino:av:uno  ~/Documents/Arduino/Blink/

Error during build: Platform 'arduino:av' not found: platform not installed

• Does this PR introduce a breaking change, and is
  titled accordingly?

Nope.

• Other information:

Fixes #1531.

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