Add placeholder message when there's no public rooms at all

I guess we never tested this against a truly empty server yet, as the
"no search" message was empty. This would normally never show up since
the top public rooms are usually listed instead.

src/libneochat/qml/ExploreRoomsPage.qml

@@ -117,6 +117,7 @@ SearchPage {
 
     searchFieldPlaceholder: i18n("Find a room…")
     noResultPlaceholderMessage: i18nc("@info:label", "No public rooms found")
+    noSearchPlaceholderMessage: i18nc("@info:label", "No public rooms")
 
     Component {
         id: manualRoomDialog