power: return the unsure state from the shellcmd driver

[Poseidonas]

docs/config.rst describes check-on as:

Should return 0 if power is on, 1 if it is off. Any other return code is interpreted as error.

The error branch in ShellCmdPowerController.status() evaluates self.POWER_UNSURE without returning it, so the method falls through and yields None.

That None reaches the user rather than staying internal. grpc/servicer.py:478 sends str(result or ''), so mtda-cli target status prints an empty line where the documentation says it should report an unknown state.

Driving the controller with subprocess.run patched, before and after:

check-on exit=  0   ON     -> 'ON'
check-on exit=  1   OFF    -> 'OFF'
check-on exit=  2   None   -> ''      after: '???'
check-on exit=127   None   -> ''      after: '???'

The change is the missing return.

I scanned the rest of the tree with the AST for the same shape — a bare self.X expression statement where a return was meant — and this is the only occurrence.

flake8 is clean with the exclusions from tox.ini. I could not run scripts/test-using-docker here (the archlinux image has no linux/arm64 manifest, and mtda-service imports systemd, which is not installable on macOS), so this rests on the isolated check above.

Found while looking at #596, which turns out to be already fixed by 9b2890e — the documentation now reads correctly, but the code no longer matches it in this one branch. Worth closing #596 if it was left open by oversight.

[chombourger]

good catch and thanks for being so thorough (e.g. use of AST to check for similar issues in the code tree)