Allow EIP binding if the current IP of the instance is null.

It's possible for myPublicIP to be null when eureka is restarted
because unbinding an EIP on Amazon results in a null IP for the
instance for a few minutes. In that case, it's ok to rebind the
new EIP.
Merge pull request #49 from mampcs/master

Allow EIP binding if the current IP of the instance is null.