[Detection Rules] Remove empty values in threshold.field array for th…

…reshold rules (#97111)

## Issues
related to elastic/detection-rules#1097
related to elastic/detection-rules#1099

## Summary

`threshold.field` is an array that currently requires at least one value. An empty string `""` was required if no value was supplied. Instead, this needs to be replaced with no string and just an empty array.
### Checklist

Delete any items that are not applicable to this PR.

- [x] Any text added follows [EUI's writing guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses sentence case text and includes [i18n support](https://github.com/elastic/kibana/blob/master/packages/kbn-i18n/README.md)

x-pack/plugins/security_solution/server/lib/detection_engine/rules/prepackaged_rules/credential_access_aws_iam_assume_role_brute_force.json

@@ -46,11 +46,9 @@
     }
   ],
   "threshold": {
-    "field": [
-      ""
-    ],
+    "field": [],
     "value": 25
   },
   "type": "threshold",
-  "version": 3
+  "version": 4
 }