sd-event: allow sd_event_source_is_enabled() to return false for NULL

sd-event: allow sd_event_source_is_enabled() to return false for NULL

This is a natural use case, and instead of defining a wrapper to do this
for us, let's just make this part of the API. Calling with NULL was not
allowed, so this is not a breaking change to the interface.

(After sd_event_source_is_enabled was originally added, we introduced
sd_event_source_disable_unref() and other similar functions which accept
NULL. So not accepting NULL here is likely to confuse people. Let's just
make the API usable with minimal fuss.)