pull_number undefined when action is running on push to master branch

Hi, we have action running fine on PRs, but when merging PR to master, the action fails with message:

```
Error: Action failed with error: Empty value for parameter 'pull_number': undefined
```

The workflow we use is just simple:
```
jobs:
check-commit:
runs-on: self-hosted-fleet
steps:
- name: Check commit message
uses: ad1992/clean-commit-action@v0.2.0
with:
repo-token: ${{ secrets.GITHUB_TOKEN }}
```