list/genlist/gengrid/toolbar: Fix - _nearest_visible_item_get() should always return an item.

Test Plan:
Bug:
- elementary_test->genlist/list/toolbar/gengrid Focus
- Resize the window to a state when no item is fully visible. only partially visible.
- Move Focus from outer widgets (Up/Down/Left/right buttons) to list/genlist/gengrid/toolbar.
- Focus goes away (size - 0). Reason: _nearest_visible_item_get returns NULL.

@fix

Reviewers: seoz, raster, woohyun

Reviewed By: raster

CC: seoz, nirajkr, anand.km

Differential Revision: https://phab.enlightenment.org/D696

legacy/elementary/src/lib/elm_gengrid.c

@@ -2171,7 +2171,7 @@ _elm_gengrid_nearest_visible_item_get(Evas_Object *obj, Elm_Object_Item *it)
                return item;
           }
      }
-   return NULL;
+   return it;
 }
 
 EOLIAN static Eina_Bool

legacy/elementary/src/lib/elm_genlist.c

@@ -2808,7 +2808,7 @@ _elm_genlist_nearest_visible_item_get(Evas_Object *obj, Elm_Object_Item *it)
                return item;
           }
      }
-   return NULL;
+   return it;
 }
 
 EOLIAN static Eina_Bool

legacy/elementary/src/lib/elm_list.c

@@ -1201,7 +1201,7 @@ _elm_list_nearest_visible_item_get(Evas_Object *obj, Elm_Object_Item *it)
                return item;
           }
      }
-   return NULL;
+   return it;
 }
 
 EOLIAN static Eina_Bool

legacy/elementary/src/lib/elm_toolbar.c

@@ -694,7 +694,7 @@ _elm_toolbar_nearest_visible_item_get(Evas_Object *obj, Elm_Object_Item *it)
                return item;
           }
      }
-   return NULL;
+   return it;
 }
 
 EOLIAN static Eina_Bool