ui: Fill service check background object for pending checks. (#24818)

.changelog/24818.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+ui: Ensure pending service check blocks are filled
+```

ui/app/styles/components/services.scss

@@ -119,6 +119,9 @@ table.health-checks {
           &.status-success {
             background-color: $nomad-green;
           }
+          &.status-pending {
+            background-color: $gray-300;
+          }
           &.status-failure {
             background-color: $red;
           }