workspace.create without a cwd failed with BAD_PARAMS when HOME was set
but empty, and a pane.split (or CLI/GUI launch) without argv tried to
spawn an empty program name when SHELL was empty. Fall back to /tmp and
sh as if the variables were unset, like the XDG and provider variables.

Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>