harden the config reader against an empty or malformed .env

holcommon's empty .env made the loader evaluate one blank line, which
tripped set -u. Verified afterwards on holcommon's real host stack: the CLI
and the make engine resolve the same five files and render a byte-identical
241-line compose config.

lib/cmd/doctor.sh

@@ -18,7 +18,7 @@ myos_cmd_doctor() {
 
   printf 'configuration:\n'
   for _f in $(myos_conf_files); do _ok "$_f" "read"; done
-  [ -f "$HOME/.config/myos/config" ] && _ok "$HOME/.config/myos/config" "read"
+  [ -f "${HOME:-}/.config/myos/config" ] && _ok "${HOME:-}/.config/myos/config" "read"
   [ -f "$WORKDIR/.env" ] && _ok "$WORKDIR/.env" "read"
   _ok ENV "$ENV"
   _ok USER "$USER"