Bug 1663941 - Do not call onAvailable if cached server resources are empty r=ochameau

Depends on D89609

Differential Revision: https://phabricator.services.mozilla.com/D89611

devtools/server/actors/targets/browsing-context.js

@@ -357,8 +357,9 @@ const browsingContextTargetPrototype = {
    * Wrapper around emit for resource forms to bail early after destroy.
    */
   _emitResourcesForm(name, resources) {
-    if (this.isDestroyed()) {
-      // Don't try to emit if the actor was destroyed.
+    if (resources.length === 0 || this.isDestroyed()) {
+      // Don't try to emit if the resources array is empty or the actor was
+      // destroyed.
       return;
     }
     this.emit(name, resources);

devtools/server/actors/watcher.js

@@ -213,15 +213,26 @@ exports.WatcherActor = protocol.ActorClassWithSpec(watcherSpec, {
    *        It may contain actor IDs, actor forms, to be manually marshalled by the client.
    */
   notifyResourceAvailable(resources) {
-    this.emit("resource-available-form", resources);
+    this._emitResourcesForm("resource-available-form", resources);
   },
 
   notifyResourceDestroyed(resources) {
-    this.emit("resource-destroyed-form", resources);
+    this._emitResourcesForm("resource-destroyed-form", resources);
   },
 
   notifyResourceUpdated(resources) {
-    this.emit("resource-updated-form", resources);
+    this._emitResourcesForm("resource-updated-form", resources);
+  },
+
+  /**
+   * Wrapper around emit for resource forms.
+   */
+  _emitResourcesForm(name, resources) {
+    if (resources.length === 0) {
+      // Don't try to emit if the resources array is empty.
+      return;
+    }
+    this.emit(name, resources);
   },
 
   /**