Intent of nil_map_merging_key_with_string_value

All 6 exact tests in this concept. Every test is marked by where it enters the hierarchy.

go.temporal.io/ 6 tests
- server/ 6 tests
  - chasm/ 2 tests
    
    lib/ 2 tests
    
    nexusoperation/ 1 test
    
    go.temporal.io/server/chasm/lib/nexusoperation/TestOperation_BuildExecutionInfo_ReturnsIsolatedSearchAttributes Inherited operation.go ×8
    
    scheduler/ 1 test
    
    go.temporal.io/server/chasm/lib/scheduler/TestScheduler_Describe_ReturnsIsolatedVisibilityMaps Inherited TestScheduler_Describe_ReturnsIsolatedVisibilityMaps
  - common/ 4 tests
    
    payload/ 4 tests
    
    TestMergeMapOfPayload/ 2 tests
    
    go.temporal.io/server/common/payload/TestMergeMapOfPayload/nil_map_merging_key_with_string_value Introduced here
    
    go.temporal.io/server/common/payload/TestMergeMapOfPayload/nil_payloads_are_filtered Inherited nil_payloads_are_filtered
    
    go.temporal.io/server/common/payload/TestFilterNilMemo Inherited payload.go ×1
    
    go.temporal.io/server/common/payload/TestFilterNilSearchAttributes Inherited TestFilterNilSearchAttributes