fix(sentry-apps): Handle empty webhook_url in external requests

[sentry]

This PR fixes the MissingSchema error that occurred when Sentry App external requests were made with an empty webhook_url.

Root Cause:
The urlparse() function, when given an empty string (''), returns a ParseResult with all components (scheme, netloc, path) empty. When subsequent URL building logic (e.g., in SelectRequester._build_url()) then attempts to construct a URL by replacing only the path or netloc, it results in a relative URL (e.g., /sentry/issues?...) that lacks a scheme. The requests library, used by safe_urlopen, then rejects this malformed URL with a MissingSchema exception.

Fix:
Early validation has been added to the _build_url() methods in SelectRequester, AlertRuleActionRequester, IssueLinkRequester, and the _prepare_stacktrace_link() method in components.py. These checks now explicitly raise a SentryAppIntegratorError if self.sentry_app.webhook_url is empty or None. This ensures that:

• The error is caught before a malformed URL is passed to the requests library.
• A more descriptive SentryAppIntegratorError is raised, providing clearer context (e.g., MISSING_URL) to the user and for internal logging, instead of the generic MissingSchema.
• The existing error handling mechanisms in the respective run() methods can gracefully process this specific error type.

Affected Files:

• src/sentry/sentry_apps/external_requests/select_requester.py
• src/sentry/sentry_apps/external_requests/alert_rule_action_requester.py
• src/sentry/sentry_apps/external_requests/issue_link_requester.py
• src/sentry/sentry_apps/components.py

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