pnfsmanager: Fix null value in diagnostic context

Adresses the problem that several name space messages add 'null'
to the diagnostic context.

PnfsId is an optional field in PnfsMessage and we thus have to check
for a null value.

Target: trunk
Request: 2.7
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6053/