Fix labelFilterParam value in issuable_list_root

What does this MR do?

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.
• epics_list_root.vue used in Epics list (behind a feature flag vue_epics_list).

Screenshots or Screencasts (strongly suggested)

Page	Before	After
Epics list	/groups/gitlab-org/-/epics?null[]=Gencell	/groups/gitlab-org/-/epics?label_name[]=Gencell
Test cases list	/gitlab-org/gitlab-shell/-/quality/test_cases?null[]=Reufunc	/gitlab-org/gitlab-shell/-/quality/test_cases?label_name[]=Reufunc

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