Do not use comparision with "is" for empty value
    
    There is a warning with python 3.8 at fedora rawhide about
    comparision with "is" while running ipa-server install:
    dirsrv_log.py:148: SyntaxWarning: "is not" with a literal. Did you mean "!="
    Removing "is not ''" as this should not be needed
    to make sure that timedata['nanosecond'] is empty.
    
    Signed-off-by: Tibor Dudlák <tdudlak@redhat.com>