Fixed lib/veewee/provider/core/box/build.rb to execute cleanly when no pre postinstall script is configured.

Fixed behaviour of 'handle_postinstall', where empty commands where being executed due to interference between the pre postinstall script code and the standard postinstall code.

This interference meant that instead of executing the configured postinstall scripts, an additional empty command will be executed after each of them. This causes no harm in the file system but may prevent subsequent scripts to finish correctly (for instance, a script may involve a reboot but the 'wait for ssh' from the 'empty command' will capture the result of the reboot, hanging the installation process).