Support empty string attributes

In May, 2020, Amazon added support to DynamoDB for empty string attributes - documentation. This major version release supports this new behavior.

In earlier iterations of ExAws.Dynamo, an empty string field would be stripped off before being written to the DB - so %{id: "1", string_field: ""} would end up writing just %{id: "1"} to the DB; retrieving that record would return %{id: "1", string_field: nil}, as this app will decode a non-existent field as nil. Following this release, the record would be written and retrieved as-is.