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"

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

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)

lib/tags.sh

@@ -6,7 +6,10 @@
 # to consul, fabio routes on the urlprefix- tags it finds there.
 
 # myos_var NAME  value of the variable named NAME, empty when unset
-myos_var() { eval "printf '%s' \"\${$1:-}\""; }
+myos_var() {
+  [ -n "${1:-}" ] || return 0
+  eval "printf '%s' \"\${$1:-}\""
+}
 
 # myos_uri SERVICE PORT [BASE_URI]
 # <service>.<base uri>, unless <SERVICE>_SERVICE[_<port>]_NAME overrides the prefix