system: provide no default for custom skeleton path

system: provide no default for custom skeleton path

Being custom means that our default one is not suitable to start with.
So there is no reason to offer it as the default path.

Add a check that it is not empty.

Add a separating empty line, for good measure, too.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@openwide.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>