The second paragraph of the PR/commit describes this; I should have made it clearer.

On OS X, opening a file with light.sh causes LT to call rebuild-argv twice. (This is a pre-existing glitch.) The first time the argstring will contain `--dir` and the desired paths, while the second time it will only contain the chromium flags set by nw. If we don't check for this second `dir`-less argstring explicitly, it fails the pred, thus skipping the processing and returning a vector containing the entire argstring as a single item.