code: @Nullable
      public ByteIterable get(@NotNull final Transaction txn, @NotNull final ByteIterable first) {
          return this.first.get(txn, first);
      }
docs: Search for the first entry in the first database. Use this method for databases configured with no duplicates.
  
  @param txn   enclosing transaction
  @param first first key.
  @return null if no entry found, otherwise the value.