gst: Fix up a bunch of GIR annotations

This is mostly on nullable return values, and some other minor ones that
I ran across.

https://bugzilla.gnome.org/show_bug.cgi?id=789319
diff --git a/gst/gstdevicemonitor.c b/gst/gstdevicemonitor.c
index 03b4621..e0b59ec 100644
--- a/gst/gstdevicemonitor.c
+++ b/gst/gstdevicemonitor.c
@@ -342,7 +342,7 @@
  * Gets a list of devices from all of the relevant monitors. This may actually
  * probe the hardware if the monitor is not currently started.
  *
- * Returns: (transfer full) (element-type GstDevice): a #GList of
+ * Returns: (transfer full) (element-type GstDevice) (nullable): a #GList of
  *   #GstDevice
  *
  * Since: 1.4