Handle empty content better

[isidorosp]

There seem to be a few edge cases where we're not correctly showing content (possibly empty) in tiles, see here as an example:
https://tiles.mechanaut.xyz/document/k2t6wyse1ukyas2ti9e68eh5lzf05d4pl815kdh3vmwwlcrsabtrq126vnl2yc

[oaksprout]

This issue has largely been addressed in #12.

Slightly extension would be to add a empty state for Content, when a document has no content, with a simple "No content" message.