Fix avro serialization of union types

[DaveWM]

This is a fix for a bug in the avro serde, where serialization is sometimes ignoring default values that appear within a union type. This is only the case when the default value is false or nil.

The problem is that when match-clj? is called for a record type (as it is within UnionType's clj->avro method), it checks whether any fields missing from the clojure data have a default value in the avro schema. To do this it calls .defaultVal, but if the default value is nil or false this returns falsey. I've therefore replaced this call with .hasDefaultValue.

Checklist

• tests
• updated CHANGELOG (the "unreleased" section)

[fc-infosec-sast]

✅ SAST Report

No issues found. Good job! 💪