Drop empty shebang documents

Previously, a shebang forced a document to be output, even if it was
  empty.
However, this conflicts with the ability to add document comments at
  the beginning of a document where the shebang is serving as a magic
  number (for example, copyright comments):
This would create two documents, one for the shebang and a second one
  for the final comment.
Now, empty documents will be dropped regardless of whether they use the
  shebang or comment syntax.