ResourceLoaderImageModule: Improve PHP 5.3 compatibility

ResourceLoaderImageModule: Improve PHP 5.3 compatibility

In PHP 5.3.x and older, the isset() check in code example below will
yield true. In PHP 5.4 and later, and HHVM, it will correctly yield
false. http://3v4l.org/8p3hm

  $options = 'foo';
  isset( $options['bar'] );

Let's not depend on this behavior here.

Change-Id: I67e83af8afe85b3ddfb5db0009759b8ac5bb7d67