BUG-8445: Guard against NPE

BUG-8445: Guard against NPE

We have observed this NPE:

[...]
Caused by: java.lang.NullPointerException
        at org.opendaylight.controller.cluster.datastore.ShardDataTree.startCanCommit(ShardDataTree.java:810)
        at org.opendaylight.controller.cluster.datastore.SimpleShardDataTreeCohort.canCommit(SimpleShardDataTreeCohort.java:105)
        at org.opendaylight.controller.cluster.datastore.ChainedCommitCohort.canCommit(ChainedCommitCohort.java:58)
        at org.opendaylight.controller.cluster.datastore.FrontendReadWriteTransaction.directCommit(FrontendReadWriteTransaction.java:384)
        at org.opendaylight.controller.cluster.datastore.FrontendReadWriteTransaction.handleModifyTransaction(FrontendReadWriteTransaction.java:527)
        at org.opendaylight.controller.cluster.datastore.FrontendReadWriteTransaction.doHandleRequest(FrontendReadWriteTransaction.java:174)
        at org.opendaylight.controller.cluster.datastore.FrontendTransaction.handleRequest(FrontendTransaction.java:141)

Which is quite weird, as the FrontendReadWriteTransaction state seems
to indicate the transaction is ready to be committed, yet ShardDataTree
does not seem to have a record of it.

While we are investigating the root cause, this patch adds an explicit
warning when this happens.

Change-Id: I2ddff76357c33d7df2b3f25a2703c69715fbd871
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit da09aa70325ef2c6f34de73b7cf10fc63410ace2)