Allow nil def in profile definition

[ethomson]

Describe the issue

If a profile uses a rule type but does not need to specify any options, then one should be able to:

repository:
  - type: stacklok/secret_scanning

but def is required, so one must:

repository:
  - type: stacklok/secret_scanning
    def: {}

To Reproduce

No response

What version are you using?

No response