Bug 1904: Handle null ThreadExecutorStatsMXBeanImpl in ShardStats

Change-Id: I4b2845a6174986dc24ea6e4e670c78906d8a6ebe
Signed-off-by: tpantelis <tpanteli@brocade.com>
Merge "Bug 1904: Handle null ThreadExecutorStatsMXBeanImpl in ShardStats"