perf(filetype): skip contents check in match() if there is no contents (#29596)

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()`).

runtime/lua/vim/filetype.lua

@@ -2598,6 +2598,9 @@ function M.match(args)
         contents = M._getlines(bufnr)
       end
     end
+
+    -- Match based solely on content only if there is any content (for performance)
+    if not (#contents == 1 and contents[1] == '') then
       -- If name is nil, catch any errors from the contents filetype detection function.
       -- If the function tries to use the filename that is nil then it will fail,
       -- but this enables checks which do not need a filename to still work.
@@ -2615,6 +2618,7 @@ function M.match(args)
       end
     end
   end
+end
 
 --- Get the default option value for a {filetype}.
 ---