AvroCoder fails on nullable Decimal types w/ Avro 1.11.3

[clairemcginty]

Given the following schema:

{
    "type": "record",
    "name": "TestRecord",
    "fields": [
         {
           "name": "nullableDecimal",
           "type": [
             "null",
             {
                 "type": "bytes",
                 "logicalType": "decimal",
                 "precision": 4,
                 "scale": 2
             }
           ]
         }
    ]
}

Scio's default AvroCoder fails to encode records with non-null values, throwing the following error:

[info]   org.apache.avro.UnresolvedUnionException: Not in union ["null",{"type":"bytes","logicalType":"decimal","precision":4,"scale":2}]: 1.00
[info]   at org.apache.avro.generic.GenericData.resolveUnion(GenericData.java:910)
[info]   at org.apache.avro.generic.GenericDatumWriter.resolveUnion(GenericDatumWriter.java:307)
[info]   at org.apache.avro.generic.GenericDatumWriter.writeWithoutConversion(GenericDatumWriter.java:157)
[info]   at org.apache.avro.specific.SpecificDatumWriter.writeField(SpecificDatumWriter.java:108)
[info]   at org.apache.avro.generic.GenericDatumWriter.writeRecord(GenericDatumWriter.java:234)
[info]   at org.apache.avro.specific.SpecificDatumWriter.writeRecord(SpecificDatumWriter.java:92)
[info]   at org.apache.avro.generic.GenericDatumWriter.writeWithoutConversion(GenericDatumWriter.java:145)
[info]   at org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:95)
[info]   at org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:82)
[info]   at org.apache.beam.sdk.extensions.avro.coders.AvroCoder.encode(AvroCoder.java:428)

(It works on Avro 1.8.2.)

This seems like it might be an issue with Avro itself (when you step through with a debugger, the GenericDatumWriter doesn't have the right conversions registered for the class), but we could still provide a workaround for it in Scio's SpecificDatumFactory?

Full repro: https://github.com/spotify/scio/compare/repro-nullable-decimal-err?expand=1

[RustedBones]

I locally rebased the repro-nullable-decimal-err branch on the PR. Test passed