Fix bug with empty feed causing nullptr dereference

If an empty URL was entered (i.e. only whitespace) then that would be
accepted and added to the database as temporary entry to be loaded and
checked later.  However, sqlite will convert this to a NULL entry.  This
would lead to Kasts not being able to retrieve that feed from the DB,
and hence not being able to create a proper object.  This would then
lead to a nullptr dereference down the line.
Note that this is a corner case: other invalid URLs are spotted
correctly and do not cause the application to crash.

BUG: 467394