709862: gtk_ui: fix crash in handle_bts_query on non-Debian systems
Fixes #709862 and its duplicate #930085.
The problem can be reproduced on Debian systems as follows:
$ reportbug -u gtk -B guug reportbug
Traceback (most recent call last):
File "/usr/bin/reportbug", line 2317, in <module>
main()
File "/usr/bin/reportbug", line 1123, in main
return iface.user_interface()
File "/usr/bin/reportbug", line 1752, in user_interface
latest_first=self.options.latest_first)
File "/usr/lib/python3/dist-packages/reportbug/ui/gtk_ui.py", line 1768, in func
args, kwargs = op.sync_pre_operation(*args, **kwargs)
TypeError: cannot unpack non-iterable NoneType object