fix: handle empty optional fields and non-interactive mode #254

Fixes #249
When all optional fields are provided via flags, the multi-select
prompt was called with an empty list, causing "Available options
can not be empty" error.
Instead of checking for empty optional_data in each command, handle
it centrally in the multi_fuzzy_select_with_key function.
This fixes the issue in:
- berg release create
- berg pull create
- berg issue create
- berg milestone create
- berg label create
Added unit tests to verify empty list behavior.
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Co-Authored-By: Claude <noreply@anthropic.com>

Fixes bug where pull create and milestone create commands would fail
in non-interactive mode with "The input device is not a TTY" error,
even when all required fields were provided via flags.
The commands were unconditionally calling multi_fuzzy_select_with_key()
for optional fields without checking if in non-interactive mode.
Changes:
- Wrap optional field selection in non-interactive check for pull create
- Wrap optional field selection in non-interactive check for milestone create
- Add integration tests for both commands in non-interactive mode
Related to #249 which fixed the same issue for other commands.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>

Changed approach based on maintainer feedback: instead of making
multi_fuzzy_select_with_key handle empty lists, check at call sites
to ensure the function is only called when there are items to select.
- Added checks in pull_request/create.rs and milestone/create.rs
- Updated multi_fuzzy_select_with_key to error with helpful message
- Removed tests for empty list behavior
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>