2.2, InfoProvider: Fix NPE

GetSpaceTokensMessage and LinkgroupDetailsMsgHandler may return null for
uninitialized fields. This causes NPEs in the respective handlers.
This patch ensures that the messages return initialized values (i.e.,
empty sets) instead of null.

Ticket: http://rt.dcache.org/Ticket/Display.html?id=8350
Acked-by:
Target: 2.2
Require-book: no
Require-notes: no
Merge pull request #658 from kschwank/fixInfoProviderNPE/2.2

Fix info provider npe/2.2