Interpret "empty" gRPC Port object as no-port in upload

[cmaglie]

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What kind of change does this PR introduce?

Makes the gRPC Upload command more relaxed in interpreting the "Port" object as a no port upload.

What is the current behavior?

An empty gRPC Port (with Address and Protocol set to the empty string) will make the upload fail. The same upload request will work as expected if the gRPC Port is left nil/undefined.

What is the new behavior?

An empty gRPC Port is treated the same as a nil/undefined Port.

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

No

Other information

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (f3ee0a8) 62.78% compared to head (bfdf4c6) 62.80%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2202      +/-   ##
==========================================
+ Coverage   62.78%   62.80%   +0.02%     
==========================================
  Files         219      219              
  Lines       19469    19469              
==========================================
+ Hits        12224    12228       +4     
+ Misses       6159     6156       -3     
+ Partials     1086     1085       -1     

Flag	Coverage Δ
unit	62.80% <100.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
commands/upload/upload.go	74.47% <100.00%> (+0.84%)	⬆️

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[per1234]

I verified this fixes arduino/arduino-ide#2089

Thanks for your work on this and the previous #2173 Cristian!