Fetchers without source configuration fails.

Custom fetchers that doesn't need a source configuration, fails per default. The problem is that if sourceForm doesn't return a form, the $node->feeds value ends up not being an array. That causes problems later when feeds does a += defaultConfig. It can be worked around by adding a sourceForm that does:

  return array(
    '#type' => 'hidden',
    '#value' => 'dummy',
  );

But this still creates an empty Feeds fieldset.

Attached patch fixes the issue, and doesn't add the fieldset if it has no content.