bluez & v4l: add basic check for nil monitor

If PipeWire is built without Bluetooth or V4L support, then

monitor = SpaDevice("api.bluez5.enum.dbus", monitor_props)

monitor = SpaDevice("api.v4l2.enum.udev", config.properties or {})

will each in their respective Lua script result in a nil monitor. These commits add a basic sanity check to avoid further using the nil variable.

Thanks-to: Pascal Flöschel (initial bug report)

Thanks-to: George Kiagiadakis george.kiagiadakis@collabora.com

Bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=994085

Bug: https://bugs.gentoo.org/813043