Intent of TestWorkerQueryEngine_IsNullString, TestWorkerQueryEngine_IsNotNullString

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
    
    matching/ 2 tests
    
    workers/ 2 tests
    
    go.temporal.io/server/service/matching/workers/TestWorkerQueryEngine_IsNotNullString Introduced here
    
    go.temporal.io/server/service/matching/workers/TestWorkerQueryEngine_IsNullString Introduced here