Suppress "variable may be used uninitialized" warning.

Buildfarm member caiman is showing this, which surprises me because
it's very late-model gcc (14.0.1) and ought to be smart enough to
know that elog(ERROR) doesn't return.  But we're likely to see the
same from stupider compilers too, so add a dummy initialization in
our usual style.

Author: tglsfdc

src/backend/parser/parse_expr.c

@@ -4276,6 +4276,7 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 			break;
 		default:
 			elog(ERROR, "invalid JsonFuncExpr op %d", (int) func->op);
+			default_format = JS_FORMAT_DEFAULT; /* keep compiler quiet */
 			break;
 	}