Incorrect handling of Base64-encoded values when producing from JSON

[soundmonster]

Base64-encoded values that start with zeroes result in empty messages when consumed from JSON:

To reproduce:

# create a topic
kafkactl create topic test_base64

# produce a value that starts with zeroes
echo '{"Key": "foo", "Value": "AAAAAAAD"}' | kafkactl produce test_base64 --input-format=json --value-encoding=base64

# produce a value that doesn't start with zeroes, here it's "foo"
echo '{"Key": "bar", "Value": "Zm9v"}' | kafkactl produce test_base64 --input-format=json --value-encoding=base64

# get contents of the topic in JSON format
kafkactl consume test_base64 --value-encoding=base64 --exit --from-beginning --output json

Output:

{
        "Partition": 0,
        "Offset": 0,
        "Value": ""
}
{
        "Partition": 0,
        "Offset": 1,
        "Value": "Zm9v"
}

This seems to be due to the fact that the Base64 decoded byte arrays are assumed to be null-terminated strings and are
truncated at first zero byte, instead of using the decoder return value that signifies the amount of bytes decoded.