app: treat an empty ("" not NULL) plug-in proc_install->menu_path as NULL

gimp_plug_in_handle_proc_install(): don't set the new procedure's menu
path nor its menu label if proc_install->menu_path is "", so we don't
run into action and menu creating confusion later. Do it here and not
later so the "" string never enters the system.

(cherry picked from commit 6e3495ff6969c2c35458292c7ff76fa72913b851)