_flatpak.py: Don't try to pass a NULL branch to construct a FlatpakRemoteRef. In libflatpak, even though FlatpakRef branch property defaults to NULL, any attempt to set it assumes the value passed is non-NULL, resulting in a segfault when performing a string search. Assume 'master' if a broken .flatpakref file doesn't provide a Branch field. This is a guess, but the only reasonable value to pass. Fixes linuxmint/mintinstall#448.