Always initialize VariantCall return_type.

The return_type is used by the GDScript parser (and possibly other
scripting languages), so it MUST be initialized at least.
It could be initialized to Variant::NIL in release, but I see no reason
for not setting the actual value.
See similar issue in 95dfa5b .

core/variant_call.cpp

@@ -155,9 +155,7 @@ struct _VariantCall {
 		funcdata.default_args = p_defaultarg;
 		funcdata._const = p_const;
 		funcdata.returns = p_has_return;
-#ifdef DEBUG_ENABLED
 		funcdata.return_type = p_return;
-#endif
 
 		if (p_argtype1.name) {
 			funcdata.arg_types.push_back(p_argtype1.type);