protected boolean isOptional(Schema.Field field, boolean isTopLevelField) {
    return false;
  }
Given a schema field, determine if it is an optional field. There could be cases where a field
is considered as optional even if it is marked as non-nullable in the schema. The producer could be filling in this
field and hence need not be specified in the query and hence is optional. Typically, such fields are
the top level fields in the schema.
@param field schema field
@param isTopLevelField if it is top level field in the schema
@return if the field is optional
samza-sql/src/main/java/org/apache/samza/sql/avro/AvroTypeFactoryImpl.java