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/libcharon/encoding/payloads/encrypted_payload.c

@@ -1023,7 +1023,7 @@ encrypted_fragment_payload_t *encrypted_fragment_payload_create()
 				.get_length = _frag_get_length,
 				.add_payload = _frag_add_payload,
 				.remove_payload = (void*)return_null,
-				.generate_payloads = nop,
+				.generate_payloads = (void*)nop,
 				.set_transform = _frag_set_transform,
 				.get_transform = _frag_get_transform,
 				.encrypt = _frag_encrypt,