Fix generate-docker-image-tags API calls (#59)

Only `get-pr-number` if a shortcut token exists

Author: Michael Vu

generate-docker-image-tags/action.yml

@@ -63,6 +63,7 @@ runs:
     - name: Get PR Number
       id: get-pr-number
       shell: bash
+      if: ${{ inputs.shortcut-api-token }}
       env:
         GITHUB_TOKEN: ${{ github.token }}
       run: |