[7.8] fix 400 error on initial signals search (#70618) (#70663)

* fix 400 error on initial signals search (#70618)

### Summary

On initial render of the SIEM pages, a 400 error was showing for POST http://localhost:5601/api/detection_engine/signals/search. This PR is a temporary fix for this bug. This initial call is being used to populate the Last alert text that shows at the top of a number of the pages. The reason the size was 0 is because we weren't interested in the signals themselves, just the timestamp of the last alert. Teamed up with @XavierM and it seems to us that the issue is the server side validation. It may be Hapi misreading the 0 as false or our updated validation not accepting size 0.

Author: yctercero

x-pack/plugins/siem/public/pages/detection_engine/components/signals_info/query.dsl.ts

@@ -30,7 +30,7 @@ export const buildLastSignalsQuery = (ruleId: string | undefined | null) => {
             : queryFilter,
       },
     },
-    size: 0,
+    size: 1,
     track_total_hits: true,
   };
 };