Fix NPE in save command

Fixes

java.lang.NullPointerException: null
    at org.dcache.cells.UniversalSpringCell.ac_save(UniversalSpringCell.java:455) ~[dcache-core-2.8.1-SNAPSHOT.jar:2.8.1-SNAPSHOT]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.7.0_51]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) ~[na:1.7.0_51]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_51]
    at java.lang.reflect.Method.invoke(Method.java:606) ~[na:1.7.0_51]
    at dmg.util.command.AcCommandExecutor.execute(AcCommandExecutor.java:157) ~[cells-2.8.1-SNAPSHOT.jar:2.8.1-SNAPSHOT]
    at dmg.util.CommandInterpreter$CommandEntry.execute(CommandInterpreter.java:385) ~[cells-2.8.1-SNAPSHOT.jar:2.8.1-SNAPSHOT]
    at dmg.util.CommandInterpreter.execute(CommandInterpreter.java:281) ~[cells-2.8.1-SNAPSHOT.jar:2.8.1-SNAPSHOT]
    at dmg.util.CommandInterpreter.command(CommandInterpreter.java:215) ~[cells-2.8.1-SNAPSHOT.jar:2.8.1-SNAPSHOT]
    at dmg.cells.nucleus.CellAdapter.executeLocalCommand(CellAdapter.java:1025) ~[cells-2.8.1-SNAPSHOT.jar:2.8.1-SNAPSHOT]
    at dmg.cells.nucleus.CellAdapter.messageArrived(CellAdapter.java:952) ~[cells-2.8.1-SNAPSHOT.jar:2.8.1-SNAPSHOT]
    at dmg.cells.nucleus.CellNucleus$DeliverMessageTask.innerRun(CellNucleus.java:1070) ~[cells-2.8.1-SNAPSHOT.jar:2.8.1-SNAPSHOT]
    at dmg.cells.nucleus.CellNucleus$AbstractNucleusTask.run(CellNucleus.java:975) ~[cells-2.8.1-SNAPSHOT.jar:2.8.1-SNAPSHOT]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [na:1.7.0_51]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [na:1.7.0_51]
    at dmg.cells.nucleus.CellNucleus$1.run(CellNucleus.java:687) [cells-2.8.1-SNAPSHOT.jar:2.8.1-SNAPSHOT]
    at java.lang.Thread.run(Thread.java:744) [na:1.7.0_51]

Affects all services that use UniversalSpringCell and do not have a default
setup file.

Target: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/6576/
(cherry picked from commit 4f7d02d9c2e5d6e3eeded68acf00352689905bd4)