Fix empty cuts behavior

[iglunchadze]

Diplodoc docs do not forbid empty cuts. But building a document with an cut which is made empty (intentionally or not, e.g. when none of conditional operators inside the cut return true) results in an error, seemingly irrelevant:

ERR Cut must be closed in /w-endcut.md

See the gist for examples of this and #10.

Empty cuts should either be allowed or result in their own error.