_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.

Author: mtwebster

usr/lib/python3/dist-packages/mintcommon/installer/_flatpak.py

@@ -1026,7 +1026,7 @@ def _pkginfo_from_file_thread(cache, file, callback):
                 except GLib.Error as e:
                     if e.code == GLib.KeyFileError.KEY_NOT_FOUND:
                         warn("Installer: flatpak - flatpakref file doesn't have a Branch key, maybe nightly or testing.")
-                        branch = None
+                        branch = "master"
 
                 remote_name = _get_remote_name_by_url(fp_sys, url)