Only attempt to render content with Tilt if delegate has any content

Calling Tilt with a empty Textile doc cause error on JRuby. This can
happend when a file only contain front matter metadata and has no
real content to render on it's own.

Simply check if delegate has any content, if it does call the tilt template.
Merge pull request #378 from aslakknutsen/redcloth_empty

Only attempt to render content with Tilt if delegate has any content.

Thanks Aslak