Intent of empty_invoker_is_invalid

All 3 exact tests in this concept. Every test is marked by where it enters the hierarchy.

go.temporal.io/ 3 tests
- server/ 3 tests
  - chasm/ 3 tests
    
    lib/ 3 tests
    
    scheduler/ 3 tests
    
    TestExecuteTask_Validate/ 3 tests
    
    go.temporal.io/server/chasm/lib/scheduler/TestExecuteTask_Validate/backing-off_start_is_invalid Inherited backing-off_start_is_invalid
    
    go.temporal.io/server/chasm/lib/scheduler/TestExecuteTask_Validate/empty_invoker_is_invalid Introduced here
    
    go.temporal.io/server/chasm/lib/scheduler/TestExecuteTask_Validate/running_start_(RunId_set)_is_invalid Inherited running_start_(RunId_set)_is_invalid