Fix repository deletion warning

Summary:
Since 6166da75 the invalid/default Definition has a name ("None"), so
this check also triggers on that one, all the time. Instead check the
definition file name, that doesn't exist for an invalid Definition.

See also D19432, which triggered this.

Reviewers: cullmann, dhaumann

Reviewed By: cullmann, dhaumann

Subscribers: kossebau, cullmann, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D19453