Slack action, add none-empty input verification

Author: s3rj1k

Diff for: .github/actions/slack/action.yml

@@ -14,6 +14,7 @@ runs:
   steps:
     - name: Send custom JSON data to Slack workflow
       id: slack
+      if: inputs.CHANNEL != '' && inputs.MESSAGE != ''
       uses: slackapi/[email protected]
       with:
         payload: |