[Security Solution] Fix coverage overview console errors (elastic#178126

)

**Fixes: elastic#164846

## Summary

This PR fixes MITRE ATT&CK® Coverage Overview dashboard console errors when the page loads or filter is changed.

## Details

An error message `Exception list is null when it never should be. This indicates potentially that saved object migrations did not run correctly. Returning empty exception list` appears each time MITRE ATT&CK® Coverage Overview dashboard is loaded or filter is changed (which lead to data reloading).

If one tried to find the source of the console error message it lead to [kibana/x-pack/plugins/security_solution/server/lib/detection_engine/rule_types/saved_object_references/inject_exceptions_list.ts](https://github.com/elastic/kibana/blob/6cb73aaec15b230a65d2e17a9d6ef970a1061709/x-pack/plugins/security_solution/server/lib/detection_engine/rule_types/saved_object_references/inject_exceptions_list.ts#L30-L33). It happens whenever there is an attempt to inject rule exceptions from SO references. This operations happens for every rule wrapped in `securityRuleTypeWrapper` and registered in `alerting` plugin. Whenever such rule if fetched via `rulesClient.find()` it invokes registered `useSavedObjectReferences.injectReferences` finally invoking mentioned above `inject_exceptions_list.ts`. Coverage Overview API endpoint fetches only necessary set of rule fields (`name`, `enabled`, `params.threat`) to reduce transferring unnecessary data. It's not hard to guess that `params.exceptionsList` is missed  and it ends up to be `undefined` (it's always an array in a SO) when the validation happens in `inject_exceptions_list.ts`. So it leads to the warning message appearing.

This PR removes redundant `exceptionsList` injection validation.

(cherry picked from commit cd16d03)

x-pack/plugins/security_solution/server/lib/detection_engine/rule_types/saved_object_references/inject_exceptions_list.test.ts

@@ -46,15 +46,14 @@ describe('inject_exceptions_list', () => {
     ).toEqual<FuncReturn>([]);
   });
 
-  test('logs expect error message if the exceptionsList is undefined', () => {
-    injectExceptionsReferences({
-      logger,
-      exceptionsList: undefined as unknown as RuleParams['exceptionsList'],
-      savedObjectReferences: mockSavedObjectReferences(),
-    });
-    expect(logger.error).toBeCalledWith(
-      'Exception list is null when it never should be. This indicates potentially that saved object migrations did not run correctly. Returning empty exception list'
-    );
+  test('returns empty array given undefined', () => {
+    expect(
+      injectExceptionsReferences({
+        logger,
+        exceptionsList: undefined as unknown as RuleParams['exceptionsList'],
+        savedObjectReferences: mockSavedObjectReferences(),
+      })
+    ).toEqual([]);
   });
 
   test('returns empty array given an empty array for "exceptionsList"', () => {

x-pack/plugins/security_solution/server/lib/detection_engine/rule_types/saved_object_references/inject_exceptions_list.ts

@@ -26,14 +26,8 @@ export const injectExceptionsReferences = ({
   logger: Logger;
   exceptionsList: RuleParams['exceptionsList'];
   savedObjectReferences: SavedObjectReference[];
-}): RuleParams['exceptionsList'] => {
-  if (exceptionsList == null) {
-    logger.error(
-      'Exception list is null when it never should be. This indicates potentially that saved object migrations did not run correctly. Returning empty exception list'
-    );
-    return [];
-  }
-  return exceptionsList.map((exceptionItem, index) => {
+}): RuleParams['exceptionsList'] =>
+  (exceptionsList ?? []).map((exceptionItem, index) => {
     const savedObjectReference = getSavedObjectReferenceForExceptionsList({
       logger,
       index,
@@ -54,4 +48,3 @@ export const injectExceptionsReferences = ({
       return exceptionItem;
     }
   });
-};