two-fer: "unlock_by": null (#483)
two-fer: "unlock_by": null (#483)

Not "unlock_by": "null"  This crashes `configlet tree .`.  See exercism/configlet#102
robkeim/xcsharp,GKotfis/csharp,robkeim/xcsharp,exercism/xcsharp,GKotfis/csharp,ErikSchierboom/xcsharp,ErikSchierboom/xcsharp,exercism/xcsharp