Allow environment variable to be set to empty value.

[przepompownia]

As discussed in #9093 Ghostty cannot empty or undefine environment variables using env option. We have to use workarounds (like ghostty() { TMUX= command ghostty "$@"; } for bash).

Actual example: export TMUX3=1; ghostty --env=TMUX3= leaves TMUX3 value unmodified.