Preserve null collections in effective YAML

internal/config/effective_projection.go

@@ -74,6 +74,20 @@ func projectionJSON(projection effectivePipelineProjection) ([]byte, error) {
 	return data, nil
 }
 
+func projectionValueTree(value any) (any, error) {
+	data, err := json.Marshal(value)
+	if err != nil {
+		return nil, fmt.Errorf("encode typed pipeline projection: %w", err)
+	}
+	decoder := json.NewDecoder(bytes.NewReader(data))
+	decoder.UseNumber()
+	var root any
+	if err := decoder.Decode(&root); err != nil {
+		return nil, fmt.Errorf("decode effective pipeline projection: %w", err)
+	}
+	return root, nil
+}
+
 func projectionValueRecords(projection effectivePipelineProjection) ([]EffectivePipelineValueRecord, error) {
 	detailed, err := projectionDetailedRecords(projection)
 	if err != nil {
@@ -87,15 +101,13 @@ func projectionValueRecords(projection effectivePipelineProjection) ([]Effective
 }
 
 func projectionDetailedRecords(value any) ([]projectedPipelineValue, error) {
-	data, err := json.Marshal(value)
+	decoded, err := projectionValueTree(value)
 	if err != nil {
-		return nil, fmt.Errorf("encode typed pipeline projection: %w", err)
+		return nil, err
 	}
-	decoder := json.NewDecoder(bytes.NewReader(data))
-	decoder.UseNumber()
-	var root map[string]any
-	if err := decoder.Decode(&root); err != nil {
-		return nil, fmt.Errorf("decode effective pipeline projection: %w", err)
+	root, ok := decoded.(map[string]any)
+	if !ok {
+		return nil, fmt.Errorf("effective pipeline projection must be a mapping")
 	}
 	var records []projectedPipelineValue
 	appendProjectionRecords(&records, nil, root)