Fix labelFilterParam value in issuable_list_root

When set as `null`, the `labelFilterParam` passed from
issuable_list_root.vue to issuable_item.vue does not
fallback to the default value of `label_name`. Instead,
we should use `undefined` so the prop falls back properly.

This bug affects test_case_list_root.vue used in Test
cases list and epics_list_root.vue used in Epics list
(behind a feature flag vue_epics_list).

Changelog: fixed
EE: true

app/assets/javascripts/issuable_list/components/issuable_list_root.vue

@@ -134,7 +134,7 @@ export default {
     labelFilterParam: {
       type: String,
       required: false,
-      default: null,
+      default: undefined,
     },
     isManualOrdering: {
       type: Boolean,