Problem: `vim.filetype.match()` tries to match on contents even if there
is no contents (empty buffer or `{''}` explicit contents).
This results in extra avoidable execution duration for cases.
It matters, for example, when trying to match filetype based solely
on file name (which still needs `contents` or `buf` to properly match
earlier in the code path).
Solution: skip matching based solely on contents if it is `{''}`. This
works because:
- Matching solely on content is done after any user-configured
`vim.filetype.add()` hooks.
- All default matching on content might depend on supplied path
*only* if there is non-empty content (like in
`require('vim.filetype.detect').match_from_hashbang()`).
dc04ef2a20