DefaultStepContext.get should directly handle certain types

[jglick]

We do not expect CpsStepContext.doGet to ever return non-null for these so why even ask? May solve test flakes introduced by jenkinsci/pipeline-input-step-plugin#145.

[dwnusbaum]

This looks fine to me and we have considered it previously. I think it is worth running PCT against it first though to check for any unexpected consequences.

[dwnusbaum]

Oh also looking at a local patch I had for this, it might be worth updating the doGet Javadoc with something like this:

     * <p>Note that some basic types are handled directly by {@link #get} and cannot be overridden, such as {@link Run},
     * {@link Job}, {@link FlowExecution}, and {@link FlowNode}.