fix: whitespace-only .gitignore lines no longer match every file

[cpruijsen]

A .gitignore line holding only whitespace made the whole workspace disappear from the language
server. Git treats such a line as blank and skips it, so the file looks harmless in review, and the
symptom is that every file in the project silently stops resolving.

mt:pattern in script/glob/matcher.lua read state[1] without checking it was there. A
whitespace-only line leaves the state empty, the pattern compiles to something that matches
everything, and the matcher then ignores the workspace root. This returns nil for that state
instead, which is how the caller already represents "this line contributes no pattern", so the line
is skipped exactly as git skips it.

Changelog entry included, per the repo convention of adding one under Unreleased.

Fixes #3458

[github-actions]

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