fix(sentry-apps): Prevent MissingSchema with empty webhook_url

[sentry]

This PR addresses a regression where Sentry Apps with an empty string as their `webhook_url` would cause a `MissingSchema: Invalid URL ''` error.

The root cause was that the send_and_save_webhook_request function in src/sentry/utils/sentry_apps/webhooks.py would assign url = '' if sentry_app.webhook_url was an empty string. The subsequent assert url is not None would pass, leading to _send_webhook_request('') and the MissingSchema exception.

This issue was previously fixed in PR #119122 by adding if not url: return Response() after the URL assignment. However, this guard was inadvertently removed in a later PR (#120280), causing the regression.

This fix re-introduces the if not url: return Response() guard immediately after the URL assignment (line 258) and removes the now-redundant assert url is not None (line 269). This ensures that if a SentryApp does not have a webhook URL configured (i.e., it's an empty string or None), the webhook sending is silently skipped, preventing the MissingSchema error.

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Fixes SENTRY-5HAE

_{Comment @sentry <feedback> on this PR to have Autofix iterate on the changes.}