Fix undefined framework_id error when unassigning framework

What does this MR do and why?

This MR fixes an exception raised when a project's compliance framework is unassigned when there is no compliance_framework_setting for the project. It is not straight forward to reproduce the issue, but it can be reproduced by calling the setComplianceFramework graphql mutation.

Screenshots or screen recordings

Before

After

How to set up and validate locally

• Create a top level group and create a compliance framework
• Create a project within the group
• Assign the compliance framework to one of the projects in the group (Settings -> General -> Compliance Frameworks)
• Go to graphql-explorer and execute the mutation twice and observe that there are no errors:

mutation {
  projectSetComplianceFramework(input:{projectId:"gid://gitlab/Project/<id>", complianceFrameworkId:null}) {
    project {
      complianceFrameworks {
        nodes {
          id
        }
      }
    }
  }
}

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