Fixes to dynamodb empty keys.
Fixes to dynamodb empty keys.

whummer/moto,okomestudio/moto,botify-labs/moto,okomestudio/moto,rocky4570/moto,okomestudio/moto,whummer/moto,william-richard/moto,william-richard/moto,botify-labs/moto,Brett55/moto,spulec/moto,okomestudio/moto,spulec/moto,okomestudio/moto,spulec/moto,spulec/moto,Brett55/moto,spulec/moto,rocky4570/moto,rocky4570/moto,rocky4570/moto,whummer/moto,william-richard/moto,whummer/moto,okomestudio/moto,Brett55/moto,william-richard/moto,Brett55/moto,william-richard/moto,whummer/moto,Brett55/moto,rocky4570/moto,whummer/moto,william-richard/moto,rocky4570/moto,spulec/moto,botify-labs/moto,botify-labs/moto,botify-labs/moto,botify-labs/moto,Brett55/moto