Fix HelpFormatter.write_usage with empty args

[Im-Siyoun]

Fixes #3360.

This fixes HelpFormatter.write_usage() when it is called without args.

Previously, write_usage("Program") passed an empty string to wrap_text() while using the usage prefix as initial_indent. Since there was no text to wrap, wrap_text() returned an empty string and only a blank line was written.

This change preserves the usage prefix for the empty-args case, so write_usage("Program") now writes:

"Usage: Program \n"

Tests were added to cover this behavior.

Tests run:

pytest tests/test_formatting.py -q
pytest

[Im-Siyoun]

I noticed after opening this PR that #3382 and #3383 previously attempted to address the same issue and were closed.

This PR takes a slightly different approach by preserving the existing trailing-space behavior from the usage prefix. In the empty-args case, write_usage("Program") writes "Usage: Program \n" instead of only "\n".

If this behavior is not desired, or if there is a preferred direction for this issue, I’m happy to adjust the PR or close it.