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/config.sh

@@ -36,7 +36,10 @@ myos_dotenv_parse() {
 myos_dotenv_load() {
   [ -f "$1" ] || return 0
   while IFS= read -r _kv; do
+    # an empty file still yields one empty line through the here-document
+    [ -n "$_kv" ] || continue
     _k=${_kv%%=*}
+    [ -n "$_k" ] || continue
     [ -n "$(myos_var "$_k")" ] && continue
     eval "$_k=\${_kv#*=}"
   done <<EOF