Removed code for checking whether the callback parameter is NULL in ewk_view_script_e...

Removed code for checking whether the callback parameter is NULL in ewk_view_script_execute().

[Title] Removed code for checking whether the callback parameter is NULL in ewk_view_script_execute().
[Issue#] N/A
[Problem] If the callback parameter is NULL, ewk_view_script_execute() does not work.
[Cause] If the callback parameter is NULL, ewk_view_script_execute() return false.
[Solution] If the result data from the script is not required,
           NULL might be used for the callback parameter.
           So, removed code for checking NULL for the callback parameter.

Change-Id: Idae68a37b834f7cabf0c1d8cb7ef8f7919b52283