Allow empty variables in bitbake espansion
Allow empty variables in bitbake espansion

Even if the current check remind that a variable is maybe missing, there
are use cases where it is useful to replace an empty variable.

Signed-off-by: Stefano Babic <2c2905e0c2877c7e4ce40c7425e972302953fda5@denx.de>

sbabic/meta-swupdate,sbabic/meta-swupdate