Accommodate changed pyasn1 behaviour
    
    pyasn1 versions prior to 0.2.3 indicate absent value by returning `None` object, later pyasn1 versions use
    the `noValue` sentinel object which is the basis for the `.hasValue()` method call (and .isValue property).
    
    This fix makes the code compatible with both `None` sentinel and the `.hasValue()` test call thus making it
    compatible with all reasonable pyasn1 versions in circulation.
    
    Fixes RHBZ#1489184
    Original patch by Ilya Etingof <etingof@gmail.com>
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>