Null check for identity riddle

src/lib/IdentityRiddle.vala

@@ -60,6 +60,9 @@ namespace Riddle {
         }
 
         protected override Solution? solve(uint8[] riddle) {
+            if(private_key == null || private_key.length == 0) {
+                return null;
+            }
             var message = Sodium.Asymmetric.Sealing.unseal(riddle, public_key, private_key);
             if(message == null) {
                 return null;