Support parsing optionals as positionals

[mauvilsa]

What does this PR do?

Fixes #690

Before submitting

• Did you read the contributing guideline?
• Did you update the documentation? (readme and public docstrings)
• Did you write unit tests such that there is 100% coverage on related code? (required for bug fixes and new features)
• Did you verify that new and existing tests pass locally?
• Did you make sure that all changes preserve backward compatibility?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

[codecov]

Codecov Report

Attention: Patch coverage is 90.74074% with 5 lines in your changes missing coverage. Please review.

Project coverage is 99.92%. Comparing base (745eac8) to head (2d0a74a).

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
jsonargparse/_common.py	88.00%	3 Missing ⚠️
jsonargparse/_core.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #692      +/-   ##
===========================================
- Coverage   100.00%   99.92%   -0.08%     
===========================================
  Files           22       22              
  Lines         6688     6740      +52     
===========================================
+ Hits          6688     6735      +47     
- Misses           0        5       +5     

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[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
90.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud