Many of the fields in our Avro alert schemas are default null. However, the 2.0 schemas list "null" as the second allowed data type in the union, which is not allowed by the Avro spec:
(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.)
This ticket is to update the schemas. This is a minor version bump, because any (hypothetical) alerts that specified all fields with no null values would still be readable with the union data type ordering changed.