fix: startSortKey can't pass null

[foreverneverer]

startSortKey should support pass null

  public void delRange(
      byte[] hashKey, byte[] startSortKey, byte[] stopSortKey, DelRangeOptions options, int timeout)
      throws PException {
    ScanOptions scanOptions = new ScanOptions();
    scanOptions.noValue = true;
    scanOptions.startInclusive = options.startInclusive;
    scanOptions.stopInclusive = options.stopInclusive;
    scanOptions.sortKeyFilterType = options.sortKeyFilterType;
    scanOptions.sortKeyFilterPattern = options.sortKeyFilterPattern;

    options.nextSortKey = new String(startSortKey);

public class DelRangeOptions {
  public String nextSortKey = "";

Because DelRangeOptions.nextSortKey was a string, if startSortKey was passed null, new String(null) will throw NullPointerException.

Besides, this pr update junit4 to junit5 for simplying unit code, for example, support test exception method, see junit5