LP#1668725: fix Z39.50 overlay when no profile set

LP#1668725: fix Z39.50 overlay when no profile set

This patch fix an issue where, when performing an overlay
of a record from Z39.50 in the web staff client, if no
merge profile is set, the incoming record is not displayed
in the right-hand pane of the merge dialog.

To test
-------
[1] Select an overlay target, then use the web staff
    Z39.50 search interface to find a record to overlay
    it with.
[2] Open the Z39.50 overlay dialog and verify that no
    profile is set (you may need to clear the
    eg.cat.z3950.default_merge_profile key to ensure this).
[3] Note that the incoming record from Z39.50 doesn't display
    in the right-hand pane.
[4] Apply the page, then repeat step 2. This time, the incoming
    record should be displayed.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>