[JENKINS-25155] Fix NPE with Promoted Builds

#47 makes copyartifacts define "COPYARTIFACT_BUILD_NUMBER_XXXX" also for promotions of [Promoted Builds plugin] (https://wiki.jenkins-ci.org/display/JENKINS/Promoted+Builds+Plugin).
This caused NPE as:
* Uses `Promotion Process` as a copying project.
* Fails to calculate a path of a copied project relative from a copying project as Promotion Process doesn't belong to Jenkins item tree hierarchy.

This fix does:
* Uses `AbstractProject#getRootProject()` to get a copying project.
* Added null check for calculated relative paths.