fix: Support empty string enum serialization and deserialization

[HofmeisterAn]

According to the Docker Engine API documentation, an empty string is a valid value for the RestartPolicyKind enum: "", "no", "always", "unless-stopped", and "on-failure". The System.Test.Json library, version 9 or higher, throws an exception when this value is empty.

System.InvalidOperationException
Enum type 'RestartPolicyKind' uses unsupported identifier ''. It must not be null, empty, or containing leading or trailing whitespace. Flags enums must additionally not contain commas.

The PR adds a custom JSON converter to allow converting empty string values.

• Closes InvalidOperationException serializing CreateContainerParameters instance #7