Schema.createRecord allows to create non parseable Schemas

Avro's API allows to create Schema objects that are not parseable by Avro. We probably must document this behavior in detail and deprecate the method that allows to do this for a future release. Example of creating an unparseable schema (it does not have a name and no public API to assign it a name):

Schema schema = Schema.createRecord(new ArrayList<>());
Schema parsedSchema = new Schema.Parser().parse(schema.toString());