Cannot construct nullable UDTs (type, rather than value) with SubstraitBuilder

[benbellick]

For some reason, the nullability is hard-coded to false. e.g.

substrait-java/core/src/main/java/io/substrait/dsl/SubstraitBuilder.java

Line 1510 in 370f071

return Type.UserDefined.builder().urn(urn).name(typeName).nullable(false).build();