fix(tests): add coverage for empty string rendering

ui/src/Stores/SilenceFormStore.test.js

@@ -114,6 +114,8 @@ describe("SilenceFormStore.data", () => {
   it("toAlertmanagerPayload creates payload that matches snapshot", () => {
     const group = MockGroup();
     store.data.fillMatchersFromGroup(group);
+    // add empty matcher so we test empty string rendering
+    store.data.addEmptyMatcher();
     store.data.startsAt = moment([2000, 1, 1, 0, 0, 0]);
     store.data.endsAt = moment([2000, 1, 1, 1, 0, 0]);
     store.data.createdBy = "me@example.com";

ui/src/Stores/__snapshots__/SilenceFormStore.test.js.snap

@@ -26,6 +26,11 @@ Object {
       "name": "cluster",
       "value": "(dev|prod)",
     },
+    Object {
+      "isRegex": false,
+      "name": "",
+      "value": "",
+    },
   ],
   "startsAt": "2000-02-01T00:00:00.000Z",
 }