statistics: Fix NPE

Fixes

java.lang.NullPointerException: null
        at diskCacheV111.services.PoolStatisticsV0.printTotal(PoolStatisticsV0.java:1219) ~[dcache-core-2.10.0-SNAPSHOT.jar:2.10.0-SNAPSHOT]
        at diskCacheV111.services.PoolStatisticsV0.prepareDailyHtml(PoolStatisticsV0.java:1151) ~[dcache-core-2.10.0-SNAPSHOT.jar:2.10.0-SNAPSHOT]
        at diskCacheV111.services.PoolStatisticsV0.prepareDailyHtmlFiles(PoolStatisticsV0.java:1056) ~[dcache-core-2.10.0-SNAPSHOT.jar:2.10.0-SNAPSHOT]
        at diskCacheV111.services.PoolStatisticsV0.access$1100(PoolStatisticsV0.java:119) ~[dcache-core-2.10.0-SNAPSHOT.jar:2.10.0-SNAPSHOT]
        at diskCacheV111.services.PoolStatisticsV0$HourlyRunner.run(PoolStatisticsV0.java:441) ~[dcache-core-2.10.0-SNAPSHOT.jar:2.10.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus$3.run(CellNucleus.java:707) [cells-2.10.0-SNAPSHOT.jar:2.10.0-SNAPSHOT]
        at java.lang.Thread.run(Thread.java:744) [na:1.7.0_51]

If there is no data, we cannot print a total.

Target: trunk
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6963/