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

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