When dependabot PR's are opened, mention is undefined

[ebebbington]

Describe the bug
When a dependabot PR is opened, and im using github.actor for the mention, the workflow fails with cannot read property replace of null

To Reproduce

1. Create a normal dependabot action
2. In your main CI workflow try run the slack workflow but passing in the actor:

   with:
     mention: ${{ github.actor }}

Expected behavior
The workflow should not fail, the actor should be passed in