code: @Nullable
    public static BigInteger parseBigInteger (@Nullable final String sStr, @Nonnegative final int nRadix)
    {
      return parseBigInteger (sStr, nRadix, null);
    }
docs: Parse the given {@link String} as {@link BigInteger} with the specified
  radix.
  
  @param sStr
  The String to parse. May be <code>null</code>.
  @param nRadix
  The radix to use. Must be &ge; {@link Character#MIN_RADIX} and &le;
  {@link Character#MAX_RADIX}.
  @return <code>null</code> if the string does not represent a valid value.