@Override
    public String getFieldName(int number)
    {
        return number == ID_POJO ? STR_POJO : null;
    }
Will be used if {@link RuntimeEnv#POJO_SCHEMA_ON_COLLECTION_FIELDS} is enabled and the
field is assignable from Collection.

@author David Yu
@created May 19, 2016
protostuff-runtime/src/main/java/io/protostuff/runtime/PolymorphicPojoCollectionSchema.java