Handle dorment login configuration in RundeckClientManager

[norrs]

Validate both username and password has value before trying to use passwordAuth.

An empty string leaves a valid Secret type, and login information seems
to stay from earlier plugin versions and has not been removed by the
plugin-upgrade-process. (That's my assumption)

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[norrs]

I believe maybe the correct fix is to fix the upgrade process to remove dormant login information .. but I dont know the internals of jenkins.. so this kinda works for me..

/jenkins/manage/configure:

/job/foo/configure:

I don't see why it complains about User cannot be blank or null.

And when I fill in and try to remove everything on "Login" and "Password", and ensure I only have "Auth Token" and click save.. I even manage to get stacktrace such as:

java.lang.IllegalArgumentException: Password cannot be blank or null
	at org.rundeck.client.RundeckClient$Builder.validateNotempty(RundeckClient.java:321)
	at org.rundeck.client.RundeckClient$Builder.buildFormAuth(RundeckClient.java:240)
	at org.rundeck.client.RundeckClient$Builder.passwordAuth(RundeckClient.java:187)
	at org.jenkinsci.plugins.rundeck.client.RundeckClientManager.buildClient(RundeckClientManager.java:58)
	at org.jenkinsci.plugins.rundeck.client.RundeckClientManager.<init>(RundeckClientManager.java:31)
	at org.jenkinsci.plugins.rundeck.RundeckInstanceBuilder.createClient(RundeckInstanceBuilder.java:103)
	at org.jenkinsci.plugins.rundeck.RundeckNotifier$RundeckDescriptor.getRundeckJobInstance(RundeckNotifier.java:873)
	at org.jenkinsci.plugins.rundeck.RundeckNotifier$RundeckDescriptor.newInstance(RundeckNotifier.java:644)
	at org.jenkinsci.plugins.rundeck.RundeckNotifier$RundeckDescriptor.newInstance(RundeckNotifier.java:541)

Anyhow, this PR seems to fix things for me.