Empty namespace is not allowed

[vytautas-karpavicius]

Currently, if an empty namespace field exists - parser complains with:

avro: namespace key must be non-empty or omitted

However, according to the latest specification this should be allowed.

A namespace is a dot-separated sequence of such names. The empty string may also be used as a namespace to indicate the null namespace. Equality of names (including field names and enum symbols) as well as fullnames is case-sensitive.

Parser should be relaxed, as this prohibits using it without additional hacks of removing such fields before parsing.