Fix empty SPPF node from xearley ignore carry-over (fixes #1598) When an item in to_scan was carried over past an ignored sequence with item.node=None (e.g. a prediction at ptr=0 that hasn't matched anything yet), the carry-over still allocated an empty SymbolNode and stored it in node_cache. That empty node later became the left child of a PackedNode when the terminal was finally scanned, and crashed ForestSumVisitor with "max() iterable argument is empty". Skip SymbolNode creation when item.node is None — there's nothing to merge, and new_item.node staying None makes the subsequent PackedNode have left=None, which is well-handled everywhere else.