sd-bus: make sd_bus_is_{ready,open} accept NULL

sd-bus: make sd_bus_is_{ready,open} accept NULL

We didn't document this behaviour one way or another, so I think it's
OK to change. All callers do the NULL check before callling this to avoid
the assert warning, so it seems reasonable to do it internally.

sd_bus_can_send() is similar, but there we expressly say that an
error is returned on NULL, so I didn't change it.