[csolution] Handle cmd line option --active= (empty argument)

[grasci-arm]

Accept command line option --active with empty argument, implicitly taking the first target-type as the active one.
Being --active a "string option" rather than a "boolean flag", to be correctly parsed the "empty argument" needs to be in one of the formats:

--active=
--active ""
-a ""

Note -a= is not supported. In such case = is literally the argument.

Align with Open-CMSIS-Pack/cbuild#443

[github-actions]

Test Results

    3 files     21 suites   13m 10s ⏱️
  418 tests   418 ✅ 0 💤 0 ❌
1 254 runs  1 254 ✅ 0 💤 0 ❌

Results for commit caf6bd6.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.60%. Comparing base (527049f) to head (caf6bd6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tools/projmgr/test/src/ProjMgrUnitTests.cpp	50.00%	0 Missing and 3 partials ⚠️
tools/projmgr/src/ProjMgrWorker.cpp	71.42%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2167   +/-   ##
=======================================
  Coverage   66.60%   66.60%           
=======================================
  Files         181      181           
  Lines       38377    38389   +12     
  Branches    22483    22493   +10     
=======================================
+ Hits        25560    25568    +8     
  Misses       8082     8082           
- Partials     4735     4739    +4     

Flag	Coverage Δ
projmgr-cov	80.80% <66.66%> (-0.02%)	⬇️

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

Files with missing lines	Coverage Δ
tools/projmgr/include/ProjMgr.h	100.00% <ø> (ø)
tools/projmgr/src/ProjMgr.cpp	80.20% <100.00%> (ø)
tools/projmgr/src/ProjMgrWorker.cpp	87.83% <71.42%> (-0.01%)	⬇️
tools/projmgr/test/src/ProjMgrUnitTests.cpp	80.36% <50.00%> (-0.04%)	⬇️

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