regression from warmup_fraction feature causes no-warmup sim_refs to be ignored

Commit 93c1d013 for #2755 (closed) (PR #2758) broke sim_refs: without any warmup specified, sim_refs is completely ignored.

It looks like there is not a good regression test for sim_refs, which is why this was not noticed. Adding one to the unit tests is appealing but will require #3068.