Fixed get_chore_with_assignments() to include assignment_type in dict.
This resolves the ResponseValidationError where assignment_type was
missing from API responses despite being present in the database.
When updating a chore with an empty due_date field, SQLite was throwing an error
because it only accepts Python datetime and date objects, not empty strings.
This fix checks if due_date is an empty string and converts it to None before
setting the attribute, preventing the TypeError.
Error fixed: "SQLite DateTime type only accepts Python datetime and date objects as input"
