Empty string support

[matti-avilabs]

Been running into the empty strings not supported by DynamoDB error from here quite a bit. While it might be an indication of an underlying "issue" with the data, I'm wondering if this might be outdated after coming across this article? Is it worth updating this - and if so, should the aim be to have different types of picklers for key (enforce non-empty strings) and non-key (allow non-empty strings) attributes?