Bug#911087: schroot: --preserve-environment does not preserve env vars set to ""

[Peter Maydell <pmaydell@chiark.greenend.org.uk>]

Package: schroot
Version: 1.6.10-3+deb9u1

The schroot --preserve-environment is supposed to preserve the
user's environment variables. However it does not pass through
environment variables which are set to the empty string:

mnementh$ FOO=bar schroot --preserve-environment -c buster-amd64-sbuild env | grep FOO
FOO=bar
mnementh$ FOO= schroot --preserve-environment -c buster-amd64-sbuild env | grep FOO


Compare the results without schroot:
mnementh$ FOO=bar env | grep FOO
FOO=bar
mnementh$ FOO= env | grep FOO
FOO=
indicating that 'env' distinguishes variables which are unset from
those which are set to the empty string, but that when we run env
under schroot schroot has dropped the setting of FOO.

This matters because some programs treat "set to empty string" specially.
For instance gcc uses "GCC_COLORS is set to the empty string" to mean
"don't use color in error messages", but "GCC_COLORS is unset" to mean
"use whatever the default is (for debian that means use colors)".

thanks
-- PMM