Bug 2069454 - Fix conditional uninitialized read r=ltenenbaum

Only global variables are default zero-initialized. In the scenario
below, if no messages matches the given `aActionName`, `navigate` is
tested for nullity but may be uninitialized, that's UB.

Differential Revision: https://phabricator.services.mozilla.com/D323704

dom/notification/NotificationHandler.cpp

@@ -22,7 +22,7 @@ nsresult RespondOnClick(nsIPrincipal* aPrincipal, const nsAString& aScope,
                         const IPCNotification& aNotification,
                         const nsAString& aActionName) {
   if (StaticPrefs::dom_webnotifications_navigate_enabled()) {
-    nsIURI* navigate;
+    nsIURI* navigate = nullptr;
     if (aActionName.IsEmpty()) {
       navigate = aNotification.options().navigate();
     } else {