combobox: prevent empty swallow to be shown on hover_dismiss animation

combobox: prevent empty swallow to be shown on hover_dismiss animation

Summary:
Hover dismiss animation takes 0.5 sec to complete. if the genlist item_count
becomes zero, empty swallow is shown in dismiss animation.
@fix T3124
Signed-off-by: divyesh purohit <div.purohit@samsung.com>
Test Plan:
please run combobox example from elementary_test.

Open hover by pressing on left side button of combobox or pressing "enter/space" key in entry
contained in combobox.
Type a char which is not in any genlist item , for example "a' is not presented in any genlist
item.
empty swallow shouldn't seen in dismiss animation.

Reviewers: shilpasingh, cedric, raster

Subscribers: rajeshps, govi

Maniphest Tasks: T3124

Differential Revision: https://phab.enlightenment.org/D3630

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>