streams: gst_stream_type_get_name() is not nullable

It takes an enum and only the defined values are valid to pass in here
as it's not extensible from the outside.

Add a g_return_val_if_reached() for the unreachable case and return
"invalid".

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/711>

gst/gststreams.c

@@ -543,7 +543,7 @@ gst_stream_get_property (GObject * object, guint prop_id,
  *
  * Get a descriptive string for a given #GstStreamType
  *
- * Returns: (nullable): A string describing the stream type
+ * Returns: A string describing the stream type
  *
  * Since: 1.10
  */
@@ -563,8 +563,7 @@ gst_stream_type_get_name (GstStreamType stype)
     case GST_STREAM_TYPE_TEXT:
       return "text";
     default:
-      return NULL;
+      g_return_val_if_reached ("invalid");
+      return "invalid";
   }
-
-  return NULL;
 }