Issue [1395] NullPointerException raised by ldapsearch when prompt for bind passwd

When "-w -" is passed to a command-line tool such as ldapsearch, the tool requests the bind password from standard input. OpenDS on Java 6 uses java.io.Console.readPassword method. For prior releases a utility method org.opends.server.util.PasswordReader.readPasswordUsingBackspaces is used, and this routine returns a char[] with the user-supplied password, which is used to initialize a String. In the case no password was supplied (i.e., the user just presses return at the prompt), the routine returns null, which results in an exception.  This change returns char[0] in the case of an empty password, which mimics the behavior of the Java 6 java.io.Console.readPassword, and eliminates the exception.