Improve EDITOR checks

libexec/tmuxifier-new-session

@@ -46,7 +46,7 @@ fi
 content="$(cat "$template")"
 echo "${content//\{\{SESSION_NAME\}\}/$layout_name}" > "$layout_file"
 
-if [ ! -z "$EDITOR" ]; then
+if [ -n "$EDITOR" ]; then
   exec "$EDITOR" "$layout_file"
 else
   echo "Layout file has been created, but '\$EDITOR' is not set. Please "