normalize empty CheckResults fields in stateV4

Ensure that empty check results are normalized in state serialization to
prevent unexpected state changes from being written.

Because there is no consistent empty, null and omit_empty usage for
state structs, there's no good way to create a test which will fail
for future additions.

Author: jbardin

internal/states/statefile/version4.go

@@ -414,9 +414,7 @@ func writeStateV4(file *File, w io.Writer) tfdiags.Diagnostics {
 		}
 	}
 
-	if file.State.CheckResults != nil {
-		sV4.CheckResults = encodeCheckResultsV4(file.State.CheckResults)
-	}
+	sV4.CheckResults = encodeCheckResultsV4(file.State.CheckResults)
 
 	sV4.normalize()
 
@@ -576,6 +574,11 @@ func decodeCheckResultsV4(in []checkResultsV4) (*states.CheckResults, tfdiags.Di
 }
 
 func encodeCheckResultsV4(in *states.CheckResults) []checkResultsV4 {
+	// normalize empty and nil sets in the serialized state
+	if in == nil || in.ConfigResults.Len() == 0 {
+		return nil
+	}
+
 	ret := make([]checkResultsV4, 0, in.ConfigResults.Len())
 
 	for _, configElem := range in.ConfigResults.Elems {