#3927: Don't preserve ProbeConfig from OLD_VDT_LOCATION if it's an empty file

Currently, configure_gratia preserves ProbeConfig from an
OLD_VDT_LOCATION if it exists. This is in
find_and_move_probe_config_files().

Before preserving it, we should do a bit more checking of the old
ProbeConfig. At the very least, we should verify that it's not an empty
file. If we can do something deeper to figure out if it's a valid file,
we should do that too.

-alain

(10:57:19 AM) Scot Kronenfeld: I'm looking for a solution to the problem
where empty ProbeConfig files are being preserved. In order to validate
an old ProbeConfig file, does it seem reasonable to assume that it must
contain $OLD_VDT_LOCATION
(10:57:40 AM) Chris Green: Yes.