Fix missing initializer warning in gobject/tests/ifaceproperties.c:base_object_get_type()

gobject/tests/ifaceproperties.c: In function ‘base_object_get_type’:
gobject/tests/ifaceproperties.c:321:1: error: missing initializer for field ‘value_table’ of ‘GTypeInfo’ {aka ‘const struct _GTypeInfo’}
  321 | static DEFINE_TYPE_FULL (BaseObject, base_object,
      | ^~~~~~
In file included from gobject/gobject.h:24,
                 from gobject/gbinding.h:29,
                 from glib/glib-object.h:22,
                 from gobject/tests/ifaceproperties.c:21:
gobject/gtype.h:1063:26: note: ‘value_table’ declared here
 1063 |   const GTypeValueTable *value_table;
      |                          ^~~~~~~~~~~

gobject/tests/testcommon.h

@@ -40,7 +40,8 @@ prefix ## _get_type (void)					\
 	  NULL,           /* class_data */			\
 	  sizeof (name),					\
 	  0,             /* n_prelocs */			\
-	  (GInstanceInitFunc) instance_init			\
+	  (GInstanceInitFunc) instance_init,			\
+	  (const GTypeValueTable *) NULL,			\
 	};							\
 								\
       object_type = g_type_register_static (parent_type,	\