GLocalFile: return text/plain for empty files

Previously, GLib returned text/plain for empty files.

This is important because people may want to open empty (eg:
just-created) text files with the text editor.

An unintended side-effect of b6fc1df0
caused GLib to start returning application/octet-stream instead of
text/plain for these files.

This commit is essentially a revert of that commit, with a different
solution: we move the special-case up a bit in the function and
hard-code it to text/plain.

This change does not exactly maintain the old behaviour: previously, a
"fast" lookup would have returned application/octet-stream on an empty
file and now it will return text/plain.  I consider this to be an
improvement (since we're returning better data) and don't expect it to
cause problems.

https://bugzilla.gnome.org/show_bug.cgi?id=755795