null-sink: Set latency range at the time of initialization of module.

At the time of module initialization latency range is being set so that the null-sink
would be aware of its limitations with latencies.

src/modules/module-null-sink.c

@@ -315,6 +315,8 @@ int pa__init(pa_module*m) {
         goto fail;
     }
 
+    pa_sink_set_latency_range(u->sink, 0, BLOCK_USEC);
+
     pa_sink_put(u->sink);
 
     pa_modargs_free(ma);