Was working on porting a build job to our Jenkins instance and was seeing something weird on the "display" for the job. Instead of what I expected to see, let's say MapName, I would see:

null
MapName

Well turns out that in the Groovy script it was concatenating via += on a string variable that was null, like someVar += "\n<br/>MapName", which is totally valid in this environment and treats the null value as the string null for the concatenation. I can't imagine a situation where I'd want that to be the default behavior, but I guess that's life..