BUG CSSContentParser fails if CLI tidy doesn't output anything.

In cases where CLI tidy fails to recognise HTML with errors, it doesn't
output any content. This fixes that to always output the input content
even if it couldn't completely tidy the content. It seems that tidy
class in PHP already does this, so this just fixes the CLI use of tidy
to be consistent with that.

This fix can be observed on the HHVM travis builds, as tidy isn't
available compiled into HHVM, it will fallback to the CLI tidy instead.