Yes I found a similar issue when Xvfb plugin is enabled on the job before tool installation/configuration in the jenkins (master/slave) configuration. You seem to have customtools-plugin installed which clouded this issue. And I'm unable to reproduce your exact issue with customtools-plugin installed (environment having null value).
Unfortunately the Xvfb plugin, currently at least, does not offer automatic tool installation, and you have to install Xvfb on your master/slave machines and configure the path in the jenkins configuration.
I've made a small fix so that now if no installation is configured in jenkins a warning message is placed in the console of the build.
Is it alright with you that I close this issue?
Hum actually can't reproduce on centos5:
Name: Xvfb
Directory in which to find Xvfb executable: '/usr/bin/'
Slave was able to start without NPE