make(1): do not create empty shell files in jobs mode

In a chain of dependencies such as "all: build; build: do-build", the
intermediate targets do not have any commands.  In jobs mode,
nevertheless, an empty file was created and fed to the shell.  This was
unnecessary.  See jobs-empty-commands.mk.

The case of the special command line "...", which was suggested on
current-users, is not optimized since it doesn't occur in practice.

Suggested by Mateusz Guzik on current-users:
https://mail-index.netbsd.org/current-users/2021/01/26/msg040215.html

(rillig)