Fix bug in create_installer.py

Fix case when substitution is not given.

Change-Id: Ibcddcb659153b6badd2b78c9bf79166b73941b5f
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Samuli Piippo <samuli.piippo@digia.com>