Run#run(Runner job) uses Charset charset = Computer.currentComputer().getDefaultCharset() to get the local encoding to write the build log. When Jenkins master is configured with 0 executors (delegating 100% work to slaves), currentComputer() returns null, so a NullPointerException and run is recorded as FAILED.

hudson.model.Run handleFatalBuildProblem
SEVERE: #13 failed to build and we don't even have a listener
java.lang.NullPointerException