Add Comment as content default

[lukeed]

Without it, an empty file will throw the following error:

Module build failed: No input specified: provide a file name or a source string to process

In my case, I'm loading third-party components which always include an accompanying .css file, but aren't all populated.

By adding a comment as a (harmless) fallback, empty files are easily "ignored".

[jsf-clabot]

All committers have signed the CLA.

[coveralls]

Changes Unknown when pulling 911a774 on lukeed:patch-1 into ** on webpack-contrib:master**.

[jhnns]

Thanks for the work. This commit was pulled in with #398 👍

[lukeed]

Thanks~