Intent of empty_child_first_run_id

All 2 exact tests in this concept. Every test is marked by where it enters the hierarchy.

go.temporal.io/ 2 tests
- server/ 2 tests
  - service/ 2 tests
    
    history/ 2 tests
    
    TestEngine2Suite/ 2 tests
    
    TestRecordChildExecutionCompleted_ChildFirstRunId/ 2 tests
    
    go.temporal.io/server/service/history/TestEngine2Suite/TestRecordChildExecutionCompleted_ChildFirstRunId/child_first_run_id_is_set Inherited child_first_run_id_is_set
    
    go.temporal.io/server/service/history/TestEngine2Suite/TestRecordChildExecutionCompleted_ChildFirstRunId/empty_child_first_run_id Introduced here