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/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;