Fix error for enums: blank string ("") or "mro" as option name

[alpolishchuk]

• fixes Driver fail on enums with empty string #147

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Ensure PR doesn't contain untouched code reformatting: spaces, etc.
• Run flake8 and fix issues.
• Run pytest no tests failed. See https://clickhouse-sqlalchemy.readthedocs.io/en/latest/development.html.

[alpolishchuk]

@xzkostyan are there any chances for this PR to be approved/discussed?