Quote empty default values in help output

[yash27-lab]

Summary

• quote empty default values in help output so they render as "" instead of an empty token
• keep existing behavior for whitespace-containing values
• add a derive regression test for default_value_t on String

Fixes #4976

[epage]

Quoting from #6241 (comment)

As noted in #4976 (comment) and https://github.com/clap-rs/clap/blob/master/CONTRIBUTING.md#preparing-the-pr, my expectations for this would be

• Add any tests that are needed to show the current problem

• Refactor to consolidate to Escape

• Update Escape with an is_empty check, updating the tests so they continue to pass

In particular,

• we prefer tests be added in a prior commit, passing so that they show the current problem. The diff between commits then shows how the behavior changed
• Refactors should be split out into their own commit
• For builder-only logic, we should focus on only builder tests and not derive tests

[yash27-lab]

Thanks for the guidance. I updated this PR accordingly:\n\n- split into three commits (tests, refactor, then behavior change)\n- moved coverage to builder tests in tests/builder/help.rs\n- refactored escaping through a shared Escape helper\n- added empty-string handling in the final fix commit\n\nPlease take another look when you have a moment.

[epage]

Please clean up the commits to reflect how this should be reviewed and merged and not the development process.