Never show whitespace-only Copilot suggestions (#7623)

Fixes https://github.com/zed-industries/zed/issues/7582

Release Notes:

- Fixed a bug that caused Copilot to suggest leading indentation even
after the user accepted/discarded a suggestion
([#7582](https://github.com/zed-industries/zed/issues/7582))

Co-authored-by: Thorsten Ball <thorsten@zed.dev>
Co-authored-by: Bennet <bennetbo@gmx.de>