Fixed java.lang.IllegalArgumentException: Target cannot be null!. This was caused by the node.masterAddress being null.
This can happen after a cluster merge in the window of time between masterAddresses being cleared and repopulated.
Merge pull request #4130 from atlassian/nullpointerexception-in-hasongoingmigrationmaster-master

Fixed java.lang.IllegalArgumentException: Target cannot be null!. This was caused by the node.masterAddress being null.