Ensure variable contains a string before using it as URL

nmdc_server/cli.py

@@ -146,7 +146,7 @@ def require_setting(name: str):
 
     # Post a message to Slack if a Slack webhook URL is defined.
     # Reference: https://api.slack.com/messaging/webhooks#posting_with_webhooks
-    if settings.slack_webhook_url_for_ingester not in [None, ""]:
+    if isinstance(settings.slack_webhook_url_for_ingester, str):
         click.echo("Posting message to Slack.")
         response = requests.post(
             settings.slack_webhook_url_for_ingester,