Ticket 47752 - Don't add unhashed password mod if we don't have an unhashed value
    
    When performing a modify operation to replace the userpassword with a pre-hashed
    value, the modify code adds a LDAPMod that replaces the "unhashed#user#password"
    attribute with no values. While this doesn't cause any harm inside DS itself, it
    is not the correct behavior. We should only add a LDAPMod for the unhashed password
    if we actually have an unhashed value available.
    
    https://fedorahosted.org/389/ticket/47752
    
    Reviewed by mreynold@redhat.com
    
    (cherry picked from commit ccc61a3a589a47844a7e6eeb13f87e1351637618)