Avoid emptry entries in project files filter list

Summary:
Some project file manager plugins like KDevGenericManager do not have a list
of project file name filters. They need to be skipped when accumulating the
lists of file name filters instead of adding empty entries.

Test Plan:
Use "Open/Import Project..." action when running in environment with KDE_FULL_SESSION
being set and select the dropdown list of the filter selection.
No more empty entries after this patch.

Reviewers: #kdevelop, mwolff

Reviewed By: #kdevelop, mwolff

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D11912