fix: pad narrower alternatives in maybe_placeholders mode

[mmustafasenoglu]

Fixes #1629

When maybe_placeholders is enabled and an alternative within an optional group matches fewer symbols than the widest alternative in the same group, the children list is left short.

For example, with !start: "a" ["b" | "c" "d"]:

Lark('!start: "a" ["b" | "c" "d"]', maybe_placeholders=True).parse("ab")
# Before: Tree('start', ['a', 'b'])
# After:  Tree('start', ['a', 'b', None])

The absent case (parse("a")) already returned 3 elements (['a', None, None]) because empty_indices padded it correctly. However, matched narrower alternatives like parse("ab") were left short at 2 elements.

Root cause: Rules without empty_indices (representing matched alternatives) were not being padded to match the widest alternative's width.

Fix: In ParseTreeBuilder._init_builders, detect the target width from rules whose empty_indices contain trailing Nones (indicating an optional group was absent at the end). Then pad narrower alternatives that have no empty_indices with trailing Nones to match that target width.

All existing tests pass (Earley, LALR, standalone, etc.).

Co-authored-by: opencode opencode@opencode.ai

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