Avoid activating panel from an empty search

Avoid activating a panel when doing an empty search i.e. clicking ENTER without having entered any text in the search entry.

We should do nothing in this case, we fixed it by detecting for this case when an activation happens when there's still no search results view in place.

Closes #2119 (closed)

Also another fix for bug mentioned here:

Fix visibility check for rows and avoid empty searches
When pressing ENTER (i.e. activating) a search with no results ("No results found") the first Wifi panel was being activating, that's wrong because cc_panel_list_activate() should activate the first visible panel, but there are no visible panels at the moment (No results found page is shown)

The problem was the visibility check in cc_panel_list_activate() was incomplete and so it wrongly thought panels were app visible.

Part of #2119 (closed) (comments 4th and 5th)