Protect against the case where there is no private key, which results
in a NPE when trying to compare in pemKeyEquals. This seems to happen
particularly when the plugin has been upgraded, and it's trying to find
existing key matches for keys with a username/password, but no private
key.

This is possibly the root cause of JENKINS-20332.


Signed-off-by: Nigel Magnay <nigel.magnay@gmail.com>