Ensure that XFAFactory.prototype.isValid returns a boolean value

Considering the name of the method, and how it's actually being used, you'd expect it to return a boolean value.
Given how it's currently being used this inconsistency doesn't cause any issues, however we should still fix this.

Author: Snuffleupagus

src/core/xfa/factory.js

@@ -43,7 +43,7 @@ class XFAFactory {
   }
 
   isValid() {
-    return this.root && this.form;
+    return !!(this.root && this.form);
   }
 
   /**