Bugfix: Make sure property time comparison works correctly.

Bugfix: Make sure property time comparison works correctly.

Hsiu-Khuern Tang writes:

    I find that doing a tags search for SCHEDULED or DEADLINE turns
    up headings that do not have any schedule or deadlines.

    Using the example from
    http://article.gmane.org/gmane.emacs.orgmode/10274:

    #+SEQ_TODO: NEXT WAITING | DONE
    #+STARTUP: overview

    * DONE Test1
       CLOSED: [2009-01-07 Wed 12:26]

    * NEXT Test2
       DEADLINE: <2009-01-28 Wed>

    * Test3

    If I type

       C-c \ +DEADLINE<="<2009-01-28>" <RET>

    all three headlines are selected!  I expected to match the second
    headline only.

Indeed, this exposes an error in the time comparison functions
which would take a empty time stamp to mean 0.  This commit does
fix the bug.