That’s what xdgmime uses for zero-sized files (see `XDG_MIME_TYPE_EMPTY`).

Historically, GLib explicitly used `text/plain` for empty files, to
ensure they would open in a text editor. But `text/plain` is not really
correct for an empty file: the content isn’t text because there is no
content. The file could eventually become something else when written
to.

Text editors which want to be opened for new, empty files should add
`application/x-zerosize` to their list of supported content types.

Users who want to set a handler for `application/x-zerosize` on their
desktop should use
```sh
gio mime application/x-zerosize  # to see the current handler
gio mime application/x-zerosize org.gnome.gedit.desktop  # to set it
```

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2777