Add Enum automated tests

[HEROgold]

• use classvar for null_values, and use them
• remove or is none check, as we set value to None when it should.
• fix Optional datatype not being counted as optional, skipping if statement.
• Add and use hypothesis tests for generated Enum values.
• use hypothesis to generate value and None values. Fixes Ensure test_optional_string always test for null values