Cast uses of return_*(), nop() and enumerator_create_empty()

As described in the previous commit, GCC 15 uses C23 by default and that
changes the meaning of such argument-less function declarations.  So
whenever we assign such a function to a pointer that expects a function
with arguments it causes an incompatible pointer type warning.  We
could define dedicated functions/callbacks whenever necessary, but this
seems like the simpler approach for now (especially since most uses of
these functions have already been cast).

src/charon-nm/nm/nm_handler.c

@@ -195,7 +195,7 @@ nm_handler_t *nm_handler_create()
 		.public = {
 			.handler = {
 				.handle = _handle,
-				.release = nop,
+				.release = (void*)nop,
 				.create_attribute_enumerator = _create_attribute_enumerator,
 			},
 			.create_enumerator = _create_enumerator,