use strict equality test

Using `!= null` is a bit confusing: it took me a moment to realise that this would also be true if the value is `undefined`, which (I presume) is the value we should test for. Using non-strict equal tests is often discouraged for such reasons.

src/redux-query-sync.js

@@ -69,7 +69,7 @@ function ReduxQuerySync({
 
     function handleLocationUpdate(location) {
         // Support history v5
-        if (location.location != null) {
+        if (location.location !== undefined) {
             location = location.location;
         }