Fix AccountLimitsConfig error prone error

The following error was fixed:
  error: [ImpossibleNullComparison] This value cannot be null, and comparing it to null may be misleading.
    return Optional.ofNullable(rateLimits.row(type));
                              ^

Change-Id: Iffc891f138b70bbfc38c004f1081476e4c7569ec
1 file changed