`set -u` doesn't permit bash arrays to test -n
`set -u` doesn't permit bash arrays to test -n

Although set -u allows empty WHITELIST=() list to be set, you can't test
-n "$WHITELIST".  Looks like the intention of the commit that caused
this issue (Change-ID Iad2d490c86dceab148ea9ab08f457c49a5d5352e) is to
set a policy to prevent future use of bugs due to unbound variables.

Thanks StevenK for pairing with me and helping me hone my bash skills.

Change-Id: I2349c990ba53b0aaecd5119812582c7c848f1e4e
Closes-Bug: #1319987

rdo-management/diskimage-builder,zioproto/diskimage-builder,eantyshev/diskimage-builder,eantyshev/diskimage-builder,JioCloud/diskimage-builder,NaohiroTamura/diskimage-builder,openstack/diskimage-builder,zioproto/diskimage-builder,switch-ch/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,varunarya10/diskimage-builder,NaohiroTamura/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,rdo-management/diskimage-builder,varunarya10/diskimage-builder,JioCloud/diskimage-builder,takahashinobuyuki/diskimage-builder,switch-ch/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder