As part of our work on the deterministic executor, in c4e37dc we fixed a
bug that occurred when there were more CPUs than paths to fuzzy-match
on.

However, that introduced another bug that caused Zed to panic when
trying to calculate how many paths should be fuzzy-matched by each
available worker thread for worktrees that didn't contain any file.

This commit bails out early in `fuzzy::match_paths` if the vector of
paths to search is empty.