avoid errors when the size of `input_ids` passed to...

avoid errors when the size of `input_ids` passed to `PrefixConstrainedLogitsProcessor` is zero (#36489)

* avoid errors when the size of `input_ids` passed to PrefixConstrainedLogitsProcessor is zero

* use more reasonable process

* avoid early return

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Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>