Fix function-valued properties in propertyBagObserver

A property in the property bag we want to observe may be a function, in
which case we don't want to treat it as a primitive and create a
toPrimitiveObserver for it.

(Also handle the null case, in which we should fall through to return a
toPrimitiveObserver.)

harness/temporalHelpers.js

@@ -1669,7 +1669,7 @@ var TemporalHelpers = {
         if (result === undefined) {
           return undefined;
         }
-        if (typeof result === "object") {
+        if ((result !== null && typeof result === "object") || typeof result === "function") {
           return result;
         }
         return TemporalHelpers.toPrimitiveObserver(calls, result, `${formatPropertyName(key, objectName)}`);