1. Create a test job.
2. Set it up with Perforce for the SCM.
3. In the Advanced section, set the Windows System Drive and System Root to empty strings.
4. Save the config.
5. Re-open the configuration window.
6. Note that the empty strings have been replaced with C: and C:\Windows
7. Set the Windows System Root and System Drive to strings of a single space.
8. Save the config.
9. Re-open the configuration window.
10. Note that the single space strings have been replaced with empty strings
11. Save the config again, and it's back to the C: based defaults

According to JENKINS-4908, empty strings should mean that the perforce plugin should just get the data from the environment.

As we have some slaves where the system drive is C: and some where it's D: (don't ask...) we need to rely on the empty string configuration. Unfortunately, anyone who changes the job configuration needs to also change these fields to single spaces so they will be saved correctly (as empty strings). Otherwise, there will be arbitrary failures when a D: based slave is next chosen for a build.

I've patched the source to make it behave as I would expect and tested locally - will attach...