keyboard: Fix warnings when clearing the help shortcut

(gnome-control-center:20334): GLib-CRITICAL **: g_variant_new_string: assertion 'string != NULL' failed

 #0  g_logv (log_domain=0x7fffec6a040e "GLib", log_level=G_LOG_LEVEL_CRITICAL, format=<optimized out>, args=args@entry=0x7fffffffd148) at gmessages.c:1046
 #1  0x00007fffec63ca12 in g_log (log_domain=log_domain@entry=0x7fffec6a040e "GLib", log_level=log_level@entry=G_LOG_LEVEL_CRITICAL, format=format@entry=0x7fffec6a9cbd "%s: assertion '%s' failed") at gmessages.c:1079
 #2  0x00007fffec63ca39 in g_return_if_fail_warning (log_domain=log_domain@entry=0x7fffec6a040e "GLib", pretty_function=pretty_function@entry=0x7fffec6fa860 <__FUNCTION__.5201> "g_variant_new_string", expression=expression@entry=0x7fffec6fc5f2 "string != NULL") at gmessages.c:1088
 #3  0x00007fffec66890a in g_variant_new_string (string=string@entry=0x0) at gvariant.c:1227
 #4  0x00007fffecc34dd4 in g_settings_set_string (settings=settings@entry=0x14b3640, key=key@entry=0x14caac0 "help", value=value@entry=0x0) at gsettings.c:1708
 #5  0x000000000047cd2e in settings_set_binding (value=0x0, key=0x14caac0 "help", settings=0x14b3640) at cc-keyboard-item.c:117
 #6  _set_binding (set_backend=1, value=<optimized out>, item=<optimized out>) at cc-keyboard-item.c:155
 #7  cc_keyboard_item_set_property (object=<optimized out>, prop_id=<optimized out>, value=<optimized out>, pspec=0x146e300) at cc-keyboard-item.c:204
 #8  0x00007fffec93b3eb in object_set_property (nqueue=<optimized out>, value=<optimized out>, pspec=<optimized out>, object=<optimized out>) at gobject.c:1415
 #9  g_object_set_valist (object=0x149fc00, first_property_name=0x146e300 "\320\064\231", var_args=0x7fffffffd378) at gobject.c:2159
 #10 0x00007fffec93bc0f in g_object_set (_object=0x149fc00, first_property_name=first_property_name@entry=0x53a4a6 "binding") at gobject.c:2269
 #11 0x0000000000479652 in accel_cleared_callback (cell=<optimized out>, path_string=<optimized out>, data=<optimized out>) at keyboard-shortcuts.c:1625