Fix zero length stylesheet cache files bug in case of missing CSS files

(tried #rails-contrib twice, hopefully someone here will have a look
at it)

This is a patch for a potentially nasty bug that only occurs when
deploying to production/staging.

The short of it is that a missing asset file, can cause the creating
of the asset cache file to bomb without warning, thus creating 0 byte
js/css cache files in production.

The long of it is here:

https://rails.lighthouseapp.com/projects/8994/tickets/2738

+1 and/or comments are .welcome

-christos