Avoid an empty ExpressionAttributeValues when only removing attributes on update/save

[pye2k]

For attributes that have persist_nil: false (which is the default setting). Setting their value as nil will remove them from the persisted record.

However when these are the only attributes that are in the update/save request, an empty map would be generated, which subsequently gets rejected by the service.

This fix checks for that condition and prevents empty ExpressionAttributeValues from being sent in the request.

[coveralls]

Changes Unknown when pulling 30c7038 on pye2k:master into * on aws:master*.

[awood45]

This looks good - merging, will go out with next release.