[Bookmarks Runner] Filter out empty urls, simplify, fix comment

Summary:
Bookmarks may have no title, the user is free to delete it when creating
a bookmark.
A bookmark without URL would be useless (there were nothing to refer to),
so ignore any bookmark with an empty URL.
Simplify check for Firefox specific "place:" URLs.

Test Plan:
Search for "Recent" or "place"
"Recent Tags" is still filtered out

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15356