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/stack.sh

@@ -15,7 +15,7 @@ myos_path() {
   _wd=${WORKDIR:-$PWD}
   _name=${STACK_DIR_NAME:-stack}
   _out=
-  for _d in "$_wd" "$_wd/.." "$HOME/.local/share" /usr/local/share /usr/share; do
+  for _d in "$_wd" "$_wd/.." "${HOME:-/nonexistent}/.local/share" /usr/local/share /usr/share; do
     for _c in "$_d/$_name" "$_d/myos/$_name"; do
       [ -d "$_c" ] || continue
       _c=$(cd "$_c" && pwd -P)