metadata: Fix metadata request topic list length incorrect issue (#5049)

[ProBrian]

For metadata request API version >= 9, Null array length is not correctly set, the 4 bytes int should be replaced by 1 byte varint.

Fix https://github.com/confluentinc/librdkafka/issues/5049

[confluent-cla-assistant[bot]]

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:white_check_mark: ProBrian
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