This regressed (I believe) due to 2ec41c174f.
* lisp/eshell/esh-cmd.el (eshell-resume-eval): Check for non-nil
'retval' instead of for a process list (nil is also a technically a
process list!).
* test/lisp/eshell/eshell-tests.el
(eshell-test/eshell-command/background-pipeline): Remove unnecessary
'copy-tree'.
(eshell-test/eshell-command/output-buffer/sync)
(eshell-test/eshell-command/output-buffer/async): New tests.