Fix Transformer_NonRecursive crashing when root node is discarded

lark/visitors.py

@@ -326,6 +326,8 @@ def transform(self, tree: Tree[_Leaf_T]) -> _Return_T:
             else:
                 stack.append(x)
 
+        if not stack:
+            return None     # type: ignore[return-value]
         result, = stack  # We should have only one tree remaining
         # There are no guarantees on the type of the value produced by calling a user func for a
         # child will produce. This means type system can't statically know that the final result is

tests/test_trees.py

@@ -415,6 +415,17 @@ def IGNORE_TOKEN(self, token):
             result = T().transform(copied)
             self.assertEqual(result, Tree('start', [3, 7]))
 
+    def test_transformer_discard_root(self):
+        """Transformer_NonRecursive should return None (not crash) when the root node is discarded."""
+        for base in (Transformer, Transformer_NonRecursive):
+            class T(base):
+                def start(self, children):
+                    return Discard
+
+            tree = Tree('start', [Token('A', 'x')])
+            result = T().transform(tree)
+            self.assertIsNone(result)
+
     def test_merge_transformers(self):
         tree = Tree('start', [
             Tree('main', [