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.)

src/network/netdev/wireguard.c

@@ -405,7 +405,7 @@ static int peer_resolve_endpoint(WireguardPeer *peer) {
                 /* Not necessary to resolve the endpoint. */
                 return 0;
 
-        if (event_source_is_enabled(peer->resolve_retry_event_source) > 0)
+        if (sd_event_source_get_enabled(peer->resolve_retry_event_source, NULL) > 0)
                 /* Timer event source is enabled. The endpoint will be resolved later. */
                 return 0;