This commit fixes serving of newly added files

This commit handles the issue in serve command which cannot host
new files. It is worth noting that new file content won't appear
until the tags are added.

Fixed #19
> I thought this fix can solve this issue, but there is still some unobvious behaviour

Yep. I think we don't need such fallbacks as defaulting to empty metadata/content. In my vision, we should make a contract - the data should be valid. In that case on higher level we can say that:

> the document is in either valid (built successful) or invalid (raised exception) state.

so we can catch exceptions where we're building docs and warn user that the doc is invalid and was skipped.