code: public StrBuilder appendSeparator(final String standard, final String defaultIfEmpty) {
          final String str = isEmpty() ? defaultIfEmpty : standard;
          if (str != null) {
              append(str);
          }
          return this;
      }
docs: Appends one of both separators to the StrBuilder.
  If the builder is currently empty it will append the defaultIfEmpty-separator
  Otherwise it will append the standard-separator
  
  Appending a null separator will have no effect.
  The separator is appended using {@link #append(String)}.
  <p>
  This method is for example useful for constructing queries
  <pre>
  StrBuilder whereClause = new StrBuilder();
  if(searchCommand.getPriority() != null) {
  whereClause.appendSeparator(" and", " where");
  whereClause.append(" priority = ?")
  }
  if(searchCommand.getComponent() != null) {
  whereClause.appendSeparator(" and", " where");
  whereClause.append(" component = ?")
  }
  selectClause.append(whereClause)
  </pre>
  
  @param standard the separator if builder is not empty, null means no separator
  @param defaultIfEmpty the separator if builder is empty, null means no separator
  @return this, to enable chaining
  @since 2.5