devinfo sysctl handler: Do not write zero-length strings in to sbuf twice

[wulf]

Summary

This fixes missing PnPinfo and location strings in devinfo output for
devices with no assigned drivers.

Test Plan

run devinfo -v and check PnPinfo and location strings of any child of ACPI bus which do not have a assigned driver.

[wulf]

Committed as e3dd8ed77b4e7d8fda12ec80b91d89e8460b64f8. I forgot to add tags to commit message to close this review automatically.