dev: gcdb: Fix slave node availability check
dev: gcdb: Fix slave node availability check

Some panel config sets the slave node to NULL and some sets
it to an empty string when the node is not available. Fix the
slave panel node availability check, while generating the
command sting.

Change-Id: Ia0de3ca14572bfa27223145b0da53331bcca9102

efidroid/lk,CanarySolutions/little-kernel,CanarySolutions/little-kernel,efidroid/lk,CanarySolutions/little-kernel,efidroid/lk,CanarySolutions/little-kernel,efidroid/lk,efidroid/lk,CanarySolutions/little-kernel