Avoid displaying a wrong value for "Free Disk Space"

[Drachenkaetzchen]

Requested feature:

Display "unknown" if the free disk space cannot be determined instead of displaying 0.1kB.

Solves the following problem:

When users see "0.1kB space free", they immediately panic and think their card is full. However, a misconfiguration of the gcode files path can lead to this, which is more likely.

Additional information:

As discussed with meteyou on the Voron Discord

Maybe it could be possible to check the root dir array. Pls write a FR on GitHub and I will check this.