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.