ag not printing empty files with -l

[otto-dev]

Empty files that are part of the searchable subset of files should still be listed.

ag can deduct that no query will match an empty file, without opening the file, but the file should still be listed as "searchable".

Would be great if it was possible to list empty files. There was #956 but this forces one to use -u, which in my case results in a ton of garbage files in the output (which is not what I want).

[otto-dev]

@ggreer I opened a simple PR for this