Fix issue adding schedule to an existing dast profile

What does this MR do and why?

Describe in detail what your merge request does and why.

Fix the error 500 when adding a schedule to an existing DAST profile.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. create a new DAST::Profile without an schedule

user = User.first # root user
project = Project.first # find project with dast enabled
profile = Dast::Profile.find_by(project: project.id)

dast_profile = ::Dast::Profile.create!(
  project: project,
  name: 'n21a2aaaaa',
  description: 'adad',
  branch_name: project.default_branch_or_main,
  dast_site_profile: profile.dast_site_profile,
  dast_scanner_profile: profile.dast_scanner_profile
)

2. Add the schedule using GraphiQL Explorer

mutation dastProfileUpdate($input: DastProfileUpdateInput!) {
  dastProfileUpdate(input: $input) {
    dastProfile {
      editPath
    },
    pipelineUrl
    errors
  }
}

{
  "input": {
    "dastScannerProfileId": "gid://gitlab/DastScannerProfile/15",
    "dastSiteProfileId": "gid://gitlab/DastSiteProfile/15",
    "id": "gid://gitlab/Dast::Profile/112",
    "name": "A4.122",
    "runAfterUpdate": true,
    "dastProfileSchedule": {
      "startsAt": "Thu, 12 Aug 2021 06:48:50.845382000 UTC +00:00",
      "active": true,
      "timezone": "America/New_York",
      "cadence": {
        "duration": 1,
        "unit": "DAY"
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.