Intent of a_is_null, a_is_not_null

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
  - common/ 2 tests
    
    persistence/ 2 tests
    
    visibility/ 2 tests
    
    store/ 2 tests
    
    sql/ 2 tests
    
    TestSQLQueryConverter_ConvertIsExpr/ 2 tests
    
    go.temporal.io/server/common/persistence/visibility/store/sql/TestSQLQueryConverter_ConvertIsExpr/a_is_not_null Introduced here
    
    go.temporal.io/server/common/persistence/visibility/store/sql/TestSQLQueryConverter_ConvertIsExpr/a_is_null Introduced here