LUCENE-10059: Additional fix to handle n_best backtrace

[jimczi]

This commit handles possible empty backtrace when using the nbest option. The general case (best path) was fixed in a previous commit so this change is a follow up to handle all cases.

[mocobeta]

I'm sorry for the late reply, looks good to me, thanks @jimczi ! There are conflicts due to the recent refactoring #805, would you mind if I adjust this and merge to main?

Old parse() method in JapaneseTokenizer and KoreanTokenizer has been integrated into org.apache.lucene.analysis.morph.Viterbi.forward() in analysis-common module, so this change can be applied to it and we no longer need to maintain two parse() methods (in main).

[mikemccand]

@jimczi -- is this still relevant? Thanks.

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