Remove empty Eshell commands when parsing

Remove empty Eshell commands when parsing

This improves the logic for copying/stealing handles when constructing
the command form: now, we should always steal the handles for the last
real command, even if there were some trailing semicolons.

* lisp/eshell/esh-arg.el (eshell-parse-delimiter): Be stricter about
parsing so that things like "& &" aren't parsed as a single "&&" token.

* lisp/eshell/esh-cmd.el (eshell-parse-command): Get the commands in
reverse, and remove any nil commands.
(eshell-split-commands): Always return the trailing terms (except when
there were no terms to begin with).

* test/lisp/eshell/esh-cmd-tests.el
(esh-cmd-test/empty-background-command): New test.