fix(action): Skip notification when inputs blank

Don't call slackapi/slack-github-action when no Slack bot token or
channel ID is passed since both are necessary to issue a Slack
notification. This occurs in pull requests from forks, such as those
generated by Forking Renovate, because GitHub Actions does not allow
forks access to secrets for security purposes.

action.yaml

@@ -64,6 +64,7 @@ runs:
       shell: bash
       working-directory: "${{ github.action_path }}"
     - name: Send Slack notification.
+      if: inputs.bot-token != '' && inputs.channel-id != ''
       uses: slackapi/slack-github-action@v1.23.0
       with:
         channel-id: ${{ inputs.channel-id }}