It should be possible to specify nil or false when providing the :source-map option to cljs.build.api/build, for example, like this:
Currently that causes:
Using false has the same behavior. The alternative of conditionally assoc ing the key in works just fine, but is a tad awkward. It seems reasonably straightforward to fix - need to change that assert to check the value in the map and double-check that it's checked properly downstream. Happy to submit a patch if you'll take it.