I want to make sure that a PR can only be merged if a project is associated with that PR.

I have the following yml code to enforce that, but I don’t know which property of the pull_request object corresponds to the project itself.

name: Enforce Project on Pull Requests

on:
  workflow_call:
  pull_request:
    types: [opened, reopened, edited, assigned, unassigned, synchronize]

jobs:
  enforce-project:
    runs-on: ubuntu-latest
    steps:
      - name: Check for Assignee
        if: ${{ github.event_name == 'pull_request' && !github.event.pull_request.(something_here) }}
        run: |
          echo "Error: Pull request must have an assignee."
          exit 1

Can someone help me figure out that?

Thank you!