Update triage-unallowed-internal-changes.yml

Author: heiskr

.github/workflows/triage-unallowed-internal-changes.yml

@@ -39,7 +39,7 @@ jobs:
         id: filter
         with:
           # Base branch used to get changed files
-          base: ${{ github.event.pull_request.base.ref }}
+          base: ${{ github.event.pull_request.base.ref || github.base_ref || github.ref }}
 
           # Enables setting an output in the format in `${FILTER_NAME}_files
           # with the names of the matching files formatted as JSON array