Fix exception whilst syncing

event.getPushRules() may return null (for better or worse...).
Use client.getPushRulesForEvent which will calculate them if they
haven't already been calculated.

Fixes element-hq/element-web#10269

src/client.js

@@ -310,7 +310,7 @@ function MatrixClient(opts) {
                     break;
                 }
 
-                highlightCount += event.getPushActions().tweaks.highlight ? 1 : 0;
+                highlightCount += this.getPushActionsForEvent(event).tweaks.highlight ? 1 : 0;
             }
 
             // Note: we don't need to handle 'total' notifications because the counts