render_to_string fails with RTeX

render_to_string fails when RTeX is loaded due to a failure to return the result from render_without_rtex when not rendering PDF. This used to work in RTeX 2.0.5, but commit 28e42cbe0323f3ed96f3dc95a7c15b72181a9ced removed the else clause at the end of render_with_rtex.

If I restore the else clause, then render_to_string works, but I don't know why the change was made or what side effects the re-insertion of the clause might have (though I haven't seen any problems yet).

Note, this is not the same as ticket #3 render_to_string with rtex because I'm not trying to render PDF output to a string, just normal HTML output that isn't going through the RTeX renderer.