doc_repo: allow None for output again.
doc_repo: allow None for output again.

That was a regression

thiblahute/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc,hotdoc/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc,thiblahute/hotdoc