empty sample file not rendered in html

[austrin]

When either the sample input or sample output in a problem is empty, nothing is displayed. Instead, it should probably display a <pre> similar to the other samples, but with no content.

[austrin]

The stripping of the empty pre tag is done by tidy when post-processing the html statement. Setting the drop-empty-paras option in tidy to false does not seem to help with this (neither in the Ubuntu 16.04 version of tidy from 2009, nor in tidy 5.2.0).