elm_cnp: Do not request for selection_set when buffer is empty

Summary:
When a wayland client requests selection_set, Wayland server doesn't care and doesn't know how long source data is.
so, it cannot ignore request for selection_set about empty buffer.
therefore to avoid copying empty buffer, it need to check length of buffer before request selection_set.

Test Plan:
1. Run "elementray_test -to entry5" with wayland server.
2. Drag text and try to paste.
3. Drag empty area in entry (request for selection_set on empty buffer is rejected)
4. Try to paste (you can see text of step 2).

Reviewers: devilhorns, raster, seoz

CC: gwanglim

Differential Revision: https://phab.enlightenment.org/D1146

legacy/elementary/src/lib/elm_cnp.c

@@ -2453,6 +2453,9 @@ _wl_elm_cnp_selection_set(Evas_Object *obj, Elm_Sel_Type selection, Elm_Sel_Form
    if ((!selbuf) && (format != ELM_SEL_FORMAT_IMAGE))
      return elm_object_cnp_selection_clear(obj, selection);
 
+   if (buflen <= 0)
+     return EINA_FALSE;
+
    _wl_elm_cnp_init();
 
    win = elm_win_wl_window_get(obj);