Commit bf8c9583 changed the List/TreeStore overrides to use
insert_with_valuesv(), but supplied all columns instead of just those which are
not None. With this, None values cause warnings like

(runtests.py:12375): Gtk-WARNING **: /build/buildd/gtk+3.0-3.3.20/./gtk/gtkliststore.c:851: Unable to convert from (null) to gboolean

Update the tests to make warnings fatal, to catch this better.

Change _convert_row() to skip the None entries and return the list of not-None
columns, and use the latter instead of a simple range(n_columns). This matches
the behaviour before bf8c9583, where columns with None values were skipped
as well.

https://bugzilla.gnome.org/show_bug.cgi?id=672463