Add a branch check for default security policy config

What does this MR do and why?

Add a branch check for default security policy config

Related to: #450891 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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Before	After

How to set up and validate locally

1. Create a new project
2. Go to Secure > Policies
3. Click on New policy
4. Select Scan execution policy
5. Change to .yaml mode
6. Copy the yaml below:

type: scan_execution_policy
name: policy
description: ''
enabled: true
rules:
  - type: pipeline
    branches:
      - main
actions:
  - scan: secret_detection

7. Click on Configure with a merge request
8. Go to Code > Branches
9. Click on New branch
10. Create a new branch
11. Go to Code > Repository
12. Change to the new branch
13. Update the README.md file and commit
14. Go to Build > Pipelines
15. Verify there is no pipeline
16. Go to Code > Repository
17. Change to the main
18. Update the README.md file and commit
19. Go to Build > Pipelines
20. Verify a pipeline with the secret_detection job was created