gracefully return "undefined" if DMI is not in sysfs

[dee-fox]

• This (should) fix issue: Hardware model detection fails due to the lack of virtual DMI device in sysfs #3770

PR Description:

Wraps access to the virtual DMI device in a try/except statement to not crash if it doesn't exist.

Tests and Checks

• I have tested the code!
  

[svartkanin]

The function shouldn't return some ambiguous string but rather none

[dee-fox]

yeah that works, checked on hardware

[svartkanin]

You'll need to update the types as well and handle it on the caller side

[dee-fox]

type updated, the one place this function is called seems to not really care about nulltypes (other bits of hardware.py are called in the same way and they may also return null)