Avro-4193: [c] segfault cannot handle empty byte array

[steven-aerts]

What is the purpose of the change

When avro-c tries to encode an empty byte array it will avro_malloc(0) which on some architectures will return NULL. Make sure this is not interpreted as an error or or dereferenced causing a segfault.

Verifying this change

Extended unit tests to validate this case

Documentation

• Does this pull request introduce a new feature? no

[martin-g]

Please rebase!

[steven-aerts]

@martin-g sorry for that, rebased on main.