Fix avro schema conv for arrays of optional type for #312.

This is a simple fix for #312, although it is not correct in all cases. Rather than failing to convert the schema when an array's item type is a union of null and another type, this correctly returns a repeated type. But, the array cannot contain null values. To support null values in the array, which is allowed by the avro schema, we would need to insert an anonymous "item" type that is optional.