dev: gcdb: Fix slave panel node availability check

Based on the slave panel node availability, parameter 'none' is
added to the cmd string. Some panel configuration sets the slave
panel node to NULL when not available and few other panels set
it to an empty string. Fix the unhandled empty string case when
slave panel is not available.

Change-Id: I46a023c8a25af5a2d2731ff1078664eddec0824f
1 file changed