env: cannot handle empty-string envvar

[BenWiederhake]

The empty string is apparently a valid name for an environment variable, even though I strongly expect that it is used only for malicious purposes:

$ env =a env | grep -Fv PATH # GNU with GNU
PWD=/home/nattiff/Bilder/xperia/Kamera
LS_COLORS=
SHLVL=1
PATH=/bin:/usr/local/bin:/usr/bin:/bin
_=/bin/env
=a
$ ./target/debug/coreutils env =a env | grep -Fv PATH # uutils calling GNU
env: warning: no name specified for value 'a'
PWD=/home/nattiff/workspace/coreutils-rs
LS_COLORS=
SHLVL=1
_=./target/debug/coreutils
$ env =a foo=x ./target/debug/coreutils env | grep -Fv PATH # GNU calling uutils, demonstrating that the envvar was dropped, not skipped
PWD=/home/nattiff/workspace/coreutils-rs
LS_COLORS=
SHLVL=1
_=/bin/env
foo=x
$ env =a ./target/debug/coreutils env | grep -Fv PATH # GNU calling uutils
PWD=/home/nattiff/workspace/coreutils-rs
LS_COLORS=
SHLVL=1
_=/bin/env
$ ./target/debug/coreutils env =a ./target/debug/coreutils env | grep -Fv PATH # uutils calling uutils
env: warning: no name specified for value 'a'
PWD=/home/nattiff/workspace/coreutils-rs
LS_COLORS=
SHLVL=1
_=./target/debug/coreutils

This means two issues:

• uutils does not set the envvar, and instead writes something to stderr
• uutils does not parse the envvar, and instead skips it

Found while reviewing #6167.