Update issue custom field post function to empty value fails for checkbox fields

Issue Summary

Using the 'Update issue custom field' post function to clear the value of a checkbox field generates a generic error.

Steps to Reproduce

1. Create a checkbox custom field with at least one option
2. Add an 'Update issue custom field' post function to a workflow transition, leaving the new value empty
3. Create an issue and select a value for the custom field
4. Execute the transition

Expected Results

The selected value will be cleared and the status transition completed.

Actual Results

Status is not changed and the error message "Transition failed - Internal server error" is displayed

Workaround

Use the 'Clear field value' post function instead.