`pr_body_content`: in homebrew-cask, empty body Cask PRs are labeled as `bump-cask-pr`

[Michael Cho]

I am guessing this is an issue in the action, though, I am not sure.
Example: Homebrew/homebrew-cask#111815

Homebrew-cask's triage workflow
https://github.com/Homebrew/homebrew-cask/blob/master/.github/workflows/triage.yml#L35-L38

              }, {
                "label": "bump-cask-pr",
                "pr_body_content": "Created with `brew bump-cask-pr`"
              }

This may be due to defaulting to applying constraint on null PR body

actions/label-pull-requests/main.js

Lines 150 to 154 in 4b84c73

           if (body != null) { 
           	constraintApplies = body.match(constraint.pr_body_content) 
    } else { 
 constraintApplies = true 
    } 

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Not sure how this will impact behavior in relation to labelExists, so need someone familiar with existing logic to comment.