Fix Transformer_NonRecursive crashing when root node is discarded

[gaoflow]

Bug

Transformer_NonRecursive.transform() raises ValueError: not enough values to unpack (expected 1, got 0) when the root tree node's callback returns Discard.

from lark import Lark
from lark.visitors import Transformer_NonRecursive, Discard

class T(Transformer_NonRecursive):
    def start(self, children):
        return Discard

g = Lark('start: A+\nA: /[a-z]+/', lexer='basic')
tree = g.parse('hello')
T().transform(tree)  # ValueError: not enough values to unpack (expected 1, got 0)

The regular Transformer handles this correctly by returning None when the root is discarded (see Transformer.transform() lines 161–163).

Fix

Add the same empty-stack guard to Transformer_NonRecursive.transform() before the destructuring assignment.

Two-line change in lark/visitors.py + a regression test in tests/test_trees.py.