fix: tighten recipient_emails guard to reject empty list

send_max_budget_alert_email previously guarded with `is not None`, which
accepts `[]` and then crashes on `recipient_emails[0]` inside
_get_email_params. The current caller (_handle_multi_threshold_max_budget_alert)
already filters empty lists upstream, but the public method signature makes
no such guarantee — a future caller passing [] would hit IndexError.

Switch to truthiness so both None and [] fall through to the single-recipient
path.

enterprise/litellm_enterprise/enterprise_callbacks/send_emails/base_email.py

@@ -359,7 +359,7 @@ class BaseEmailLogger(CustomLogger):
             else "N/A"
         )
 
-        if recipient_emails is not None:
+        if recipient_emails:
             # Multi-threshold path: batch send with generic key-based greeting
             email_params = await self._get_email_params(
                 email_event=EmailEvent.max_budget_alert,