test/org/apache/pig/builtin/avro/schema/recordsWithNullableUnions.avsc

{
  "name" : "recordsWithNullableUnions",
  "namespace" : "org.apache.pig.test.builtin",
  "type" : "record",
  "fields" : [
    {"name" : "key", "type" : ["string","null"]},
    {"name" : "value", "type" : ["int","null"]}
  ]
}