transforms: Don't try to call has_child on parent nodes that are None

[mitya57]

Otherwise the build can fail with the following error:

  File "/usr/lib/python2.7/dist-packages/sphinx/environment.py", line 791, in read_doc
    pub.publish()
  File "/usr/lib/python2.7/dist-packages/docutils/core.py", line 218, in publish
    self.apply_transforms()
  File "/usr/lib/python2.7/dist-packages/docutils/core.py", line 199, in apply_transforms
    self.document.transformer.apply_transforms()
  File "/usr/lib/python2.7/dist-packages/docutils/transforms/__init__.py", line 171, in apply_transforms
    transform.apply(**kwargs)
  File "/usr/lib/python2.7/dist-packages/sphinx/transforms.py", line 124, in apply
    if has_child(node.parent, nodes.caption):
  File "/usr/lib/python2.7/dist-packages/sphinx/transforms.py", line 117, in has_child
    return any(isinstance(child, cls) for child in node)
TypeError: 'NoneType' object is not iterable

The error can be reproduced with lp:autopilot/legacy branch from Launchpad.

[birkenfeld]

Thanks!