sd-bus: standarize on NULL for empty signature in method calls

We would use sometimes "" and sometimes NULL. They are equivalent, so let's use
NULL everywhere, except for a two places in tests.

src/busctl/busctl.c

@@ -442,7 +442,7 @@ static int find_nodes(sd_bus *bus, const char *service, const char *path, Set *p
 
         r = sd_bus_call_method(bus, service, path,
                                "org.freedesktop.DBus.Introspectable", "Introspect",
-                               &error, &reply, "");
+                               &error, &reply, NULL);
         if (r < 0) {
                 printf("%sFailed to introspect object %s of service %s: %s%s\n",
                        ansi_highlight_red(),
@@ -981,7 +981,7 @@ static int introspect(int argc, char **argv, void *userdata) {
 
         r = sd_bus_call_method(bus, argv[1], argv[2],
                                "org.freedesktop.DBus.Introspectable", "Introspect",
-                               &error, &reply_xml, "");
+                               &error, &reply_xml, NULL);
         if (r < 0)
                 return log_error_errno(r, "Failed to introspect object %s of service %s: %s",
                                        argv[2], argv[1], bus_error_message(&error, r));