bus-util: treat an empty string as a NULL

Instead of ignoring empty strings retrieved via the bus, treat them as NULL, as
it's customary in systemd.

src/shared/bus-util.c

@@ -1025,7 +1025,7 @@ static int map_basic(sd_bus *bus, const char *member, sd_bus_message *m, sd_bus_
                         break;
 
                 if (isempty(s))
-                        break;
+                        s = NULL;
 
                 r = free_and_strdup(p, s);
                 break;