doc: env: clarify that empty signal args are ignored

doc: env: clarify that empty signal args are ignored

It's useful to treat empty and missing arguments differently.
Missing means all signals, while empty means no signals and
so is a no-op.  It's useful to treat empty arguments like
this, so that dynamically specified arguments like the following
are supported

  env --ignore-signals "$SIGS_TO_IGNORE"

Note `env --ignore-signals=` is treated as an empty argument.

* doc/coreutils.texi (env invocation): Empty args are treated
differently to missing arguments, so call that out explicitly.
* src/env.c (usage): Likewise.
Addresses https://bugs.debian.org/1016049