devicemonitor: don't fail when started without any filters

Just show all devices then.

gst/gstdevicemonitor.c

@@ -35,7 +35,7 @@
  * the application has set.
  *
  *
- * The basic use pattern of an iterator is as follows:
+ * The basic use pattern of a device monitor is as follows:
  * |[
  *   static gboolean
  *   my_bus_func (GstBus * bus, GstMessage * message, gpointer user_data)
@@ -348,8 +348,10 @@ gst_device_monitor_start (GstDeviceMonitor * monitor)
 
   if (monitor->priv->filters->len == 0) {
     GST_OBJECT_UNLOCK (monitor);
-    GST_WARNING_OBJECT (monitor, "No filters have been set");
-    return FALSE;
+    GST_WARNING_OBJECT (monitor, "No filters have been set, will expose all "
+        "devices found");
+    gst_device_monitor_add_filter (monitor, NULL, NULL);
+    GST_OBJECT_LOCK (monitor);
   }
 
   if (monitor->priv->providers->len == 0) {

tests/check/gst/gstdevice.c

@@ -420,8 +420,6 @@ GST_START_TEST (test_device_monitor)
 
   mon = gst_device_monitor_new ();
 
-  fail_if (gst_device_monitor_start (mon));
-
   devices = g_list_append (NULL, test_device_new ());
 
   devs = gst_device_monitor_get_devices (mon);
@@ -533,6 +531,12 @@ GST_START_TEST (test_device_monitor)
   gst_object_unref (dp);
   gst_object_unref (dp2);
   g_list_free_full (devices, (GDestroyNotify) gst_object_unref);
+
+  /* should work fine without any filters */
+  mon = gst_device_monitor_new ();
+  fail_unless (gst_device_monitor_start (mon));
+  gst_device_monitor_stop (mon);
+  gst_object_unref (mon);
 }
 
 GST_END_TEST;