fix(help): quote empty string defaults in help output

[njg7194]

Fixes #4976

Summary

When a default value is an empty string, the help output now displays [default: ""] instead of [default: ], making it clear that the default is an empty string rather than appearing to be missing.

Changes

• help_template.rs: Added check for empty strings in default value display
• possible_value.rs: Added check for empty strings in get_visible_quoted_name
• format.rs: Added check for empty strings in Escape Display impl

Before

Options:
      --text <TEXT>  [default: ]
  -h, --help         Print help

After

Options:
      --text <TEXT>  [default: ""]
  -h, --help         Print help

This is a minimal change that follows the existing pattern of using Rust debug representation for values that need quoting.

[epage]

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