repl: Empty command should be sent to eval function

This fixes a regression introduced in https://github.com/nodejs/node/pull/6171

PR-URL: https://github.com/nodejs/node/pull/11871


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>