Support for empty avro files

[jayt87]

I have some clues that this library does not support empty avro files (without body).
This causes problems when handling empty files captured from Event Hubs.

    public byte[] DecodeFixed(int size)
    {
        if (size <= 0)
        {
            throw new ArgumentOutOfRangeException("size");
        }

        var array = new byte[size];
        this.ReadAllRequiredBytes(array);
        return array;
    } 

[oluatte]

I'd like to second this as a real and urgent issue. As of now, it makes the event hub capture ---> data lake analytics use case very difficult. It doesn't make sense for such a vital use case to be hampered by an edge case.

It also doesn't help that we can't override the necessary methods on the decoder since it's internal and sealed.

Please address this issue.

Also: thank you for all your hard work on this library.