Previously, if a non-process was piped to a process, this could end up
being nil, which isn't correct. 'eshell-last-async-procs' should just
ignore non-process commands in a pipeline.
* lisp/eshell/esh-cmd.el (eshell-do-pipelines): Set 'headproc'
correctly.
* test/lisp/eshell/eshell-tests.el (eshell-test/pipe-headproc): New test.