Revert "change undefined to null in tx example"

This reverts commit cff614b.

Author: SebastienGllmt

doc/getting-started/generating-transactions.md

@@ -110,7 +110,7 @@ witnesses.set_bootstraps(bootstrapWitnesses);
 const transaction = CardanoWasm.Transaction.new(
     txBody,
     witnesses,
-    null, // transaction metadata
+    undefined, // transaction metadata
 );
 ```