Fix relative glob patterns not working for language servers (#9179)

If a language server would send us a glob pattern like `**/*.rb` or
`**/{package.json}` we'd end up ignoring it and never sending the
language server any notifications, because we try to `strip_prefix` the
projects absolute path from the pattern, BUT if that path is not in the
pattern, we'd return `None`.

This change fixes that.

Release Notes:

- Fixed language server glob patterns for file watching being ignored if
they were relative patterns.

Co-authored-by: Bennet <bennetbo@gmx.de>
Co-authored-by: Remco <djsmits12@gmail.com>