Mono/C#: Fix bindings generator with void vararg methods

Commit 4d727f1ee6b970298a7c1752ba19b49d7060c405 made it possible for vararg
methods to return void. This broke the C# bindings generator which was
assuming in one place that vararg methods always return Variant.

modules/mono/editor/bindings_generator.cpp

@@ -2066,9 +2066,11 @@ Error BindingsGenerator::_generate_glue_method(const BindingsGenerator::TypeInte
 			p_output.append(p_imethod.arguments.size() ? C_LOCAL_PTRCALL_ARGS ".ptr()" : "NULL");
 			p_output.append(", total_length, vcall_error);\n");
 
-			// See the comment on the C_LOCAL_VARARG_RET declaration
-			if (return_type->cname != name_cache.type_Variant) {
-				p_output.append("\t" C_LOCAL_RET " = " C_LOCAL_VARARG_RET ";\n");
+			if (!ret_void) {
+				// See the comment on the C_LOCAL_VARARG_RET declaration
+				if (return_type->cname != name_cache.type_Variant) {
+					p_output.append("\t" C_LOCAL_RET " = " C_LOCAL_VARARG_RET ";\n");
+				}
 			}
 		} else {
 			p_output.append("\t" CS_PARAM_METHODBIND "->ptrcall(" CS_PARAM_INSTANCE ", ");