background_tasks: empty state claims executions exist outside the search when there are none #469

Found during QA of #463. This is pre-existing.

Problem

On /admin/background-tasks/ with no executions at all, entering a search shows this empty-state text: "Executions exist outside the current search…". That is not true, because none exist.

Expected

When the unfiltered total is 0, show the plain empty state ("No executions yet"). The "outside the current search" wording should appear only when filters hide rows that do exist.