py: add_array() will not add to kv store if value is an empty array (#8774)

* gguf_writer.py: add_array() should not add to kv store if empty

* Apply suggestions from code review

I was wondering if there was a specific reason for `if val` but good to hear we can safely use `len(val == 0`

Co-authored-by: compilade <git@compilade.net>

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Co-authored-by: compilade <git@compilade.net>