fix https://github.com/sagemathinc/cocalc/issues/3280 -- de-archiver crashes (react error) when there is no file.
fix https://github.com/sagemathinc/cocalc/issues/3280 -- de-archiver crashes (react error) when there is no file.

tscholl2/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,DrXyzzy/smc,sagemathinc/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,sagemathinc/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc