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.

src/libsystemd/sd-bus/sd-bus.c

@@ -1800,7 +1800,9 @@ void bus_enter_closing(sd_bus *bus) {
 DEFINE_PUBLIC_TRIVIAL_REF_UNREF_FUNC(sd_bus, sd_bus, bus_free);
 
 _public_ int sd_bus_is_open(sd_bus *bus) {
-        assert_return(bus, -EINVAL);
+        if (!bus)
+                return 0;
+
         assert_return(bus = bus_resolve(bus), -ENOPKG);
         assert_return(!bus_pid_changed(bus), -ECHILD);
 
@@ -1808,7 +1810,9 @@ _public_ int sd_bus_is_open(sd_bus *bus) {
 }
 
 _public_ int sd_bus_is_ready(sd_bus *bus) {
-        assert_return(bus, -EINVAL);
+        if (!bus)
+                return 0;
+
         assert_return(bus = bus_resolve(bus), -ENOPKG);
         assert_return(!bus_pid_changed(bus), -ECHILD);