Simplifying access to Transactions and Blocks in syscalls

neo/SmartContract/InteropService.cs

@@ -353,8 +353,16 @@ private static bool Blockchain_GetBlock(ApplicationEngine engine)
         private static bool Blockchain_GetTransaction(ApplicationEngine engine)
         {
             byte[] hash = engine.CurrentContext.EvaluationStack.Pop().GetByteArray();
-            Transaction tx = engine.Snapshot.GetTransaction(new UInt256(hash));
-            engine.CurrentContext.EvaluationStack.Push(StackItem.FromInterface(tx));
+            if (hash.Length == 0)
+            {
+                if (!(engine.ScriptContainer is Transaction tx)) return false;
+                engine.CurrentContext.EvaluationStack.Push(StackItem.FromInterface(tx));
+            }
+            else
+            {
+                Transaction tx = engine.Snapshot.GetTransaction(new UInt256(hash));
+                engine.CurrentContext.EvaluationStack.Push(StackItem.FromInterface(tx));
+            }
             return true;
         }