dtoverlay: Make empty alias a valid string
dtoverlay: Make empty alias a valid string

dtoverlay_get_alias returns a string with no length indication. In
the case that the alias is present but empty, return an empty string.

raspberrypi/userland,6by9/userland,JamesH65/userland,6by9/userland,JamesH65/userland,JamesH65/userland,kraj/userland,raspberrypi/userland,kraj/userland,raspberrypi/userland,kraj/userland,6by9/userland