Change sent metadata topic array to be nullable (versions 1, 2 and 3)

[pdrvsky]

According to a documentation and some research in other libs I've fixed topic metadata fetching in Kafka protocol versions 1, 2 and 3. It used a v0 version, which sent an empty array not a -1 value.

[Nevon]

It's weird that the Kafka protocol guide doesn't mention anything about Metadata v0 expecting an array, but indeed the definition of a zero length array is null, so this change makes sense.