fix: Severity filter not working in case of empty ecosystem value (#188)

Fixes #186 


![image](https://github.com/user-attachments/assets/d03761db-dcf5-4bfa-b6ce-12cb8ef23f5b)

Author: kunalnagar

src/fetch-alerts.ts

@@ -21,7 +21,7 @@ export const fetchAlerts = async (
     repo: repositoryName,
     state: 'open',
     severity,
-    ecosystem,
+    ecosystem: ecosystem.length > 0 ? ecosystem : undefined,
     per_page: count,
   })
   const alerts: Alert[] = response.data.map((dependabotAlert) =>