Suppress image in email if none given

If "Header Image for Email:" is left blank in admin/settings/forward, an image is still generated in the HTML email, just with a blank 'src' field. For example, you might get the following HTML, containing the bogus <img src="" ...>

  <div id="header"><a href="http://example.com/"><img src="" border="0" alt="SiteName"></a></div>

The attached patch fixes this by suppressing the header image in the HTML email if it is blank.

This should also handle the feature request at http://drupal.org/node/51658 to allow images to be toggled off (simply leave blank).

Best wishes, Mark.