Improve NOT_FOUND response handling on ThreatMonitoring page

NOT_FOUND responses use the same code path as successful responses but
provide unexpected payload. This patch normalises payloads between 2
flows and improves this logic by letting mutator decide which code
path to use.

ee/app/assets/javascripts/threat_monitoring/store/modules/threat_monitoring_statistics/actions.js

@@ -3,7 +3,6 @@ import pollUntilComplete from '~/lib/utils/poll_until_complete';
 import httpStatusCodes from '~/lib/utils/http_status';
 import createFlash from '~/flash';
 import * as types from './mutation_types';
-import createState from './state';
 import { getTimeWindowParams } from '../../utils';
 
 export const requestStatistics = ({ commit }, timeWindowParams) => {
@@ -40,7 +39,7 @@ export const fetchStatistics = ({ state, dispatch, rootState }) => {
       // no data, but we can't distinguish between them, yet. So, just render
       // no data.
       if (error.response.status === httpStatusCodes.NOT_FOUND) {
-        dispatch('receiveStatisticsSuccess', createState().statistics);
+        dispatch('receiveStatisticsSuccess', null);
       } else {
         dispatch('receiveStatisticsError');
       }

ee/app/assets/javascripts/threat_monitoring/store/modules/threat_monitoring_statistics/mutations.js

 import * as types from './mutation_types';
+import createState from './state';
 
 export default transformFunc => ({
   [types.SET_ENDPOINT](state, endpoint) {
@@ -10,7 +11,8 @@ export default transformFunc => ({
     state.timeRange = timeRange;
   },
   [types.RECEIVE_STATISTICS_SUCCESS](state, payload) {
-    state.statistics = transformFunc(payload);
+    const stats = payload ? transformFunc(payload) : createState().statistics;
+    state.statistics = stats;
     state.isLoadingStatistics = false;
     state.errorLoadingStatistics = false;
   },

ee/spec/frontend/threat_monitoring/store/modules/threat_monitoring_statistics/actions_spec.js

@@ -140,17 +140,7 @@ describe('threatMonitoringStatistics actions', () => {
           [],
           [
             { type: 'requestStatistics', payload: expect.any(Object) },
-            {
-              type: 'receiveStatisticsSuccess',
-              payload: expect.objectContaining({
-                total: 0,
-                anomalous: 0,
-                history: {
-                  nominal: [],
-                  anomalous: [],
-                },
-              }),
-            },
+            { type: 'receiveStatisticsSuccess', payload: null },
           ],
         ));
     });

ee/spec/frontend/threat_monitoring/store/modules/threat_monitoring_statistics/mutations_spec.js

@@ -53,6 +53,31 @@ describe('threatMonitoringStatistics mutations', () => {
     it('sets errorLoadingStatistics to false', () => {
       expect(state.errorLoadingStatistics).toBe(false);
     });
+
+    describe('with null payload', () => {
+      beforeEach(() => {
+        mutations[types.RECEIVE_STATISTICS_SUCCESS](state, null);
+      });
+
+      it('sets statistics to the empty state', () => {
+        expect(state.statistics).toEqual({
+          total: 0,
+          anomalous: 0,
+          history: {
+            nominal: [],
+            anomalous: [],
+          },
+        });
+      });
+
+      it('sets isLoadingStatistics to false', () => {
+        expect(state.isLoadingStatistics).toBe(false);
+      });
+
+      it('sets errorLoadingStatistics to false', () => {
+        expect(state.errorLoadingStatistics).toBe(false);
+      });
+    });
   });
 
   describe(types.RECEIVE_STATISTICS_ERROR, () => {