Nodelist in failure report adds a `None` at the end

[vkarak]

This is very easy to reproduce:

./bin/reframe -c unittests/resources/checks/hellocheck.py -S 'prerun_cmds=set -e;foo' -r

FAILURE INFO for HelloTest
  * Expanded name: HelloTest
  * Description: C Hello World test
  * System partition: generic:default
  * Environment: builtin
  * Stage directory: /Users/karakasv/Repositories/reframe/stage/generic/default/builtin/HelloTest
  * Node list: tresa.localNone
  * Job type: local (id=80795)
  * Dependencies (conceptual): []
  * Dependencies (actual): []
  * Maintainers: ['VK']
  * Failing phase: sanity
  * Rerun with '-n /2b3e4546 -p builtin --system generic:default -r'
  * Reason: sanity error: pattern 'Hello, World\\!' not found in 'rfm_job.out'

[vkarak]

This happens only when the node name has not trailing digits.