GtkEntryAccessible: also handle entry icon tooltip NULL in notify_gtk.

GtkEntryAccessible: also handle entry icon tooltip NULL in notify_gtk.

To clear the tooltip one is to set the tooltip to NULL. Though
the GtkEntryAccessible expect this tooltip to not be NULL in
gtk_entry_accessible_notify_gtk (already handling this case
 in its _init).

Fixes:
** (epiphany:23914): CRITICAL **: atk_object_set_description: assertion
`description != NULL' failed
when epiphany g_object_set the entry icon tooltip to NULL (clear the
tooltip) in its find bar.

https://bugzilla.gnome.org/show_bug.cgi?id=695375