code: public FieldDefinition getShardingField() {
          if (!isSharded()) {
              return null;
          }
          String shardingFieldNme = getOption(CommonDefs.OPT_SHARDING_FIELD);
          if (shardingFieldNme == null) {
              return null;
          }
          return getFieldDef(shardingFieldNme);   // may return null
      }
docs: Get the {@link FieldDefinition} of the sharding-field for this table. If this table
  is not sharded, the sharding-field option has not yet been set, or the sharding
  field has not yet been defined, null is returned.
  
  @return  The {@link FieldDefinition} of the sharding-field for this table or null
  if the table is not sharded or the sharding-field has not yet been defined.