glib-mkenums: Add default comment template if none is provided

The fallback code for providing a default comment template only worked
if a template file was provided. It didn’t work if individual templates
were provided on the command line (and --comment wasn’t).

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=788990