Fix regression of M*LIB:

oplist of tuple/variant wasn't able to detect the default INIT creation
as no valid OPLIST was passed in the absence of OPLIST.
Give the explicit default OPLIST in this case.

m-tuple.h

@@ -37,7 +37,7 @@
    USAGE: TUPLE_OPLIST(name[, oplist of the first type, ...]) */
 #define TUPLE_OPLIST(...)                                          \
   M_IF_NARGS_EQ1(__VA_ARGS__)                                      \
-  (TUPLEI_OPLIST(__VA_ARGS__, () ),                                \
+  (TUPLEI_OPLIST(__VA_ARGS__, M_DEFAULT_OPLIST ),		   \
    TUPLEI_OPLIST(__VA_ARGS__ ))
 
 /* Return an array suitable for the _cmp_order function.

m-variant.h

@@ -40,7 +40,7 @@
    USAGE: VARIANT_OPLIST(name[, oplist of the first type, ...]) */
 #define VARIANT_OPLIST(...)                                        \
   M_IF_NARGS_EQ1(__VA_ARGS__)                                      \
-  (VARIANTI_OPLIST(__VA_ARGS__, () ),                              \
+  (VARIANTI_OPLIST(__VA_ARGS__, M_DEFAULT_OPLIST),		   \
    VARIANTI_OPLIST(__VA_ARGS__ ))