Fix crash in upload if the upload tool platform is missing

[cmaglie]

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

Fix a crash in upload if:

• The platform requires an upload tool from another (referenced) platform
• The referenced platform is missing

What is the current behavior?

See #2042 for more details:

$ arduino-cli compile -b arduino:avr:uno --upload arduino-cli/internal/integrationtest/testdata/bare_minimum
Sketch uses 444 bytes (1%) of program storage space. Maximum is 32256 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Maximum is 2048 bytes.
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0xaee351]

goroutine 1 [running]:
github.com/arduino/arduino-cli/arduino/cores/packagemanager.(*Explorer).GetInstalledPlatformRelease(0xc00076f600, 0xc0003ba9c0?)
        /home/megabug/Workspace/arduino-cli/arduino/cores/packagemanager/package_manager.go:576 +0x31
github.com/arduino/arduino-cli/commands/upload.runProgramAction(0xc00076f600, 0xc000721ce2?, {0x0, 0x0}, {0x0, 0x0}, {0x7ffcf7a16ea4, 0xf}, 0x40f5e8?, {0x0, ...}, ...)
        /home/megabug/Workspace/arduino-cli/commands/upload/upload.go:276 +0x97e
github.com/arduino/arduino-cli/commands/upload.Upload({0x17791f0?, 0x0?}, 0xc00029b040, {0x114bd00, 0xc00012b068}, {0x114bd00, 0xc00012b080})
        /home/megabug/Workspace/arduino-cli/commands/upload/upload.go:143 +0x445
github.com/arduino/arduino-cli/internal/cli/compile.runCompileCommand(0xc00038aa00?, {0xc0002599c0, 0x1, 0x4?})
        /home/megabug/Workspace/arduino-cli/internal/cli/compile/compile.go:268 +0x1287
github.com/spf13/cobra.(*Command).execute(0xc00038aa00, {0xc000259980, 0x4, 0x4})
        /home/megabug/Software/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:860 +0x663
github.com/spf13/cobra.(*Command).ExecuteC(0xc0002a9180)
        /home/megabug/Software/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:974 +0x3bd
github.com/spf13/cobra.(*Command).Execute(0x0?)
        /home/megabug/Software/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:902 +0x19
main.main()
        /home/megabug/Workspace/arduino-cli/main.go:31 +0xea

What is the new behavior?

arduino-cli compile -b arduino:avr:uno --upload /home/megabug/Workspace/arduino-cli/internal/integrationtest/testdata/bare_minimum
Sketch uses 444 bytes (1%) of program storage space. Maximum is 32256 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Maximum is 2048 bytes.
Error during Upload: Platform 'foo:avr' not found

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

No

Other information

Fix #2042

[codecov]

Codecov Report

Patch coverage: 72.72% and project coverage change: +0.12 🎉

Comparison is base (8cd7297) 62.86% compared to head (1b61a3f) 62.98%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2211      +/-   ##
==========================================
+ Coverage   62.86%   62.98%   +0.12%     
==========================================
  Files         218      218              
  Lines       19462    19466       +4     
==========================================
+ Hits        12235    12261      +26     
+ Misses       6145     6122      -23     
- Partials     1082     1083       +1     

Flag	Coverage Δ
unit	62.98% <72.72%> (+0.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
commands/upload/upload.go	74.68% <72.72%> (+0.21%)	⬆️

... and 1 file with indirect coverage changes

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