Scripting: prevent `callDBus` from crashing KWin on bad arguments

The FDO dbus library defaults to aborting on invalid arguments, unless the DBUS_FATAL_WARNINGS environment variable is set to 0.

BUG: 494869

https://gitlab.freedesktop.org/dbus/dbus/-/blob/master/dbus/dbus-internals.c?ref_type=heads#L242