code: public static Calendar parseCalendar(final String calendar, final String format, final TimeZone timeZone) {
          if (N.isNullOrEmpty(calendar) || (calendar.length() == 4 && "null".equalsIgnoreCase(calendar))) {
              return null;
          }
  
          return createCalendar(parse(calendar, format, timeZone));
      }
docs: Converts the specified <code>calendar</code> with the specified {@code format} to a new instance of Calendar.
  <code>null</code> is returned if the specified <code>date</code> is null or empty.
  
  @param calendar
  @param format
  @param timeZone
  @return