Avro dataset: nullable fields (unions) are in reverse order

The automatically generated Avro schema contained in the OCF header of files from ADF are unions of ` | null`, rather than `null | `, which does not follow the recommendation in the Avro spec for nullable types:

> (Note that when a default value is specified for a record field whose type is a union, the type of the default value must match the first element of the union. Thus, for unions containing "null", the "null" is usually listed first, since the default value of such unions is typically null.)

https://avro.apache.org/docs/1.8.1/spec.html#Unions

The outcome is that some libraries fail to identify these fields as nullable types, for example in Go where a nullable union of T can be mapped to a (nullable) pointer *T.