[JENKINS-45519] Fix keepUndefinedParameters option for suppressing warnings (#2939)

* Update ParametersAction.java

Pull request 2687 added a 'false' setting for this flag to prevent warning messages from being logged, but the logic doesn't match the message or the documentation. This updates the check, so that the warning message is correctly suppressed if `hudson.model.ParametersAction.keepUndefinedParameters` is set to false.

* Updated check to use optBoolean