MDL-42790 essay attempt on last: fix plain reponse mode

Plain responses, without files, were getting messed up by the
fix for MDL-39980. Something that looked like an HTML comment was being
appended.

This fix works by avoiding appending anything if there are no files. The
new unit test (which was failing before I fixed the code) confirms that
this works. The other tests should be enough to verify that there are no
regressions.