The answer depends upon the exact language of the form and, secondarily, if the employer's ability to discipline an employee is limited by a contract, collective bargaining agreement (union contract) or civil service law (for government employees).
If the employer is simply requiring you to fill out a donation form with your name and indicate that you are donating $0, with no other personal information disclosed, that has the potential to cause employees to feel uncomfortable, but would not violate the employees' legal rights.

