Fix field type in Cue.

This field is typed to be a non-nullable string, but the compiler was
ignoring this type since it was defined on the interface.  A new
version of the compiler caught this error.  Originally cr/257309718.

Change-Id: I9e7589c3b059ca39324078c867dbc27f3742a095

Author: TheModMaker

lib/text/cue.js

@@ -139,7 +139,7 @@ shaka.text.Cue = class {
      * @override
      * @exportInterface
      */
-    this.backgroundImage = null;
+    this.backgroundImage = '';
 
     /**
      * @override