(dcache-webadmin) null guard on billing service method

module: dcache-webadmin (billing)

An unexpected error in the service endpoint (billing cell) can result in the return value being null.  Rather than have a NPE reported, a guard as been added.

Target: master
Patch: http://rb.dcache.org/r/5670/
Require-notes: no
Require-book: no
Request: 2.6
Acked-by: Gerd