optional array in avro

[Philipp]

I'm wondering whether or not it is possible to have an optional array.
Let's assume a schema like this:
{ "type": "record",
  "name": "test_avro",
  "fields" : [
    {"name": "test_field_1", "type": "long"},
    {"name": "subrecord", "type": [{
      "type": "record",
      "name": "subrecord_type",
      "fields":[{"name":"field_1", "type":"long"}]
      },"null"]
    },
    {"name": "simple_array",
    "type":{
        "type": "array",
        "items": "string"
      }
    }
  ]
}

Trying to write an avro record without "simple_array" would result in a 
NPE in the datafilewriter.
For subrecord it's just fine, but when I try to define the array as 
optional:

{"name": "simple_array",
    "type":[{
        "type": "array",
        "items": "string"
      }, "null"]


It does not result in a NPE but a runtime exception:
AvroRuntimeException: Not an array schema: 
[{"type":"array","items":"string"},"null"]

Thanks.