pool: Fix -sticky option in migration module

Addresses a regression that causes the -sticky option when
used without a value to not match any files.

The issue was that the empty value of a multi-valued option
was translated to an array with the empty string rather than
the empty array.

Target: trunk
Request: 2.7
Request: 2.6
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8108
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6247/