You have described the problem exactly. The issue does seem to be with HP's version of realpath. Apparently, when the file doesn't exist, it returns NULL. I haven't had time to check the getFullPath implementations for the other platforms to see if a solution already exists.
At this point, I don't know what the correct behaviour for getFullPath is when the file doesn't exist. Maybe somebody can enlighten me. Oh, and just in case somebody is worried about stepping on my toe, I am NOT an HP expert. So if you are and want to step in, consider yourself invited. Dudley Irish