prevent exceptions with empty and- and or-qualifiers in ERXPrefixQualifierTraversal

[sgaertner]

In Wonder it is legal to create empty ERXAndQualifier and ERXOrQualifiers. This is convenient when creating complex qualifiers dynamically, which depend on user input and whether certain sub-qualifiers shall be included or not.

However when trying to prefix a qualifier structure, which contains empty ERXAndQualifier and ERXOrQualifiers, an exception is thrown due to an invalid NSRange. Sample code (both lines generate an exception):

  System.out.println(new ERXKey("prefix").dot(ERXQ.and()));
  System.out.println(new ERXKey("prefix").dot(ERXQ.and(ERXQ.equals("foo", "bar"), ERXQ.or())));

This patch solves the issue in ERXPrefixQualifierTraversal.