Rac grant form: message field is still used in validation even though is optional

[amgciadev]

Describe the bug
The message field is optional but a requirement for a not empty message is still used to disable the submit button

To Reproduce
Steps to reproduce the behavior:

1. Submit a request
2. Attempt to grant request
3. Form won't allow you to submit until there is a message even though it says it's optional

Expected behavior
A user can grant a request without entering a message

Related work
#2439