Description of problem:
If the admin has a custom 'readahead_sysconfig' snippet which is empty (because they want to avoid Beaker's default readahead fiddling in their environment), as of Beaker 0.18 this will result in invalid syntax in the kickstart %post.

Release number of selected component (if applicable):
0.18.2

How reproducible:
with admin access

Steps to Reproduce:
1. Create empty file /etc/beaker/snippets/readahead_sysconfig
2. Run a job

Actual results:
Kickstart %post contains:

if [ -f /etc/sysconfig/readahead ] ; then
fi

which is a syntax error, so %post script does not work properly.

Expected results:
No syntax error, statement should have no effect if the snippet is empty.

Additional info:
Some other snippets suffer the same problem, need to audit to find which ones.

Thanks to Jun'ichi Nomura for the original report: http://article.gmane.org/gmane.comp.systems.beaker.devel/1118