Use more optional chaining in the code-base

This patch updates a bunch of older code, that makes conditional function calls, to use optional chaining rather than `if`-blocks.

These mostly mechanical changes reduce the size of the `gulp mozcentral` build by a little over 1 kB.

src/scripting_api/app.js

@@ -100,16 +100,12 @@ class App extends PDFObject {
     const timeout = Object.create(null);
     const id = { callbackId, interval };
     this._timeoutIds.set(timeout, id);
-    if (this._timeoutIdsRegistry) {
-      this._timeoutIdsRegistry.register(timeout, id);
-    }
+    this._timeoutIdsRegistry?.register(timeout, id);
     return timeout;
   }
 
   _unregisterTimeout(timeout) {
-    if (this._timeoutIdsRegistry) {
-      this._timeoutIdsRegistry.unregister(timeout);
-    }
+    this._timeoutIdsRegistry?.unregister(timeout);
 
     const data = this._timeoutIds.get(timeout);
     if (!data) {

src/scripting_api/doc.js

@@ -950,10 +950,9 @@ class Doc extends PDFObject {
   }
 
   getPrintParams() {
-    if (!this._printParams) {
-      this._printParams = new PrintParams({ lastPage: this._numPages - 1 });
-    }
-    return this._printParams;
+    return (this._printParams ||= new PrintParams({
+      lastPage: this._numPages - 1,
+    }));
   }
 
   getSound() {

src/scripting_api/event.js

@@ -91,9 +91,7 @@ class EventDispatcher {
       } else if (id === "app" && baseEvent.name === "ResetForm") {
         for (const fieldId of baseEvent.ids) {
           const obj = this._objects[fieldId];
-          if (obj) {
-            obj.obj._reset();
-          }
+          obj?.obj._reset();
         }
       }
       return;