Fix `branch_type` feature flagging

What does this MR do and why?

Fixes two problems with the feature flagging introduced by !121679 (merged). Both problems are limited to scan execution policies of type: pipeline.

• In the case of group-level policies, the feature check compares to nil instead of a Project: !123385 (diffs, comment 1427166279)
• With the feature flag disabled, branch_type policies currently raise: !123385 (diffs, comment 1427168812)

See: #404774 (comment 1427073846)

How to set up and validate locally

• Create a group

• Create a contained project

• Disable the feature flag for the project:

  Feature.disable(:security_policies_branch_type, Project.find(-1))

• Commit the following .gitlab-ci.yml:

  dummy_job:
    script:
      - exit 0

• On the group level, navigate to Security and Compliance > Policies and create the following scan execution policy:

  type: scan_execution_policy
  name: Pipeline / Default Branch / Container Scan
  description: ''
  enabled: true
  rules:
    - type: pipeline
      branch_type: default
  actions:
    - scan: container_scanning
      variables:
        CS_IMAGE: "nginx:1"

• Navigate to CI/CD > Pipelines, run a pipeline for the main branch and verify only the dummy_job was created.

• Enable the feature flag for the project:

  Feature.enable(:security_policies_branch_type, Project.find(-1))

• Navigate to CI/CD > Pipelines, run a pipeline for the main branch and verify both dummy_job and container-scanning-0 were created.

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.

Related to #404774 (closed)