check for empty strings in prepare_items

[iamtalhaasghar]

if a column is nullable then treat empty strings as Null values.

• fixes str object has no attribute 'tzinfo' #487

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-driver.readthedocs.io/en/latest/development.html.