Change the return value for an empty search query (Issue #726)

Return an empty string instead of NULL when the search is active but there is no query.

This allows thunar_standard_view_set_searching to disable thunar_standard_view_select_after_row_deleted which calls set_cursor. We want set_cursor disabled
because it massively slows down Thunar when searching (since it gets called everytime that a row is removed from the view model).