The error message is most likely related to haxelib/neko trying to add two null strings.
I have encountered an issue before with Jenkins, since Jenkins removes HOMEDRIVE and HOMEPATH from the env-vars while doing a build. You may be encountering something similar.
Check and make sure HOMEDRIVE and HOMEPATH are set in your current shell, the re-try haxelib. If this does not work, we'll have to narrow down further.