fix corner case in merge_vars (#5320)

fixes #5319

lib/output.js

@@ -103,6 +103,7 @@ function OutputStream(options) {
 
     function make_indent(value) {
         if (typeof value == "number") return new Array(value + 1).join(" ");
+        if (!value) return "";
         if (!/^\s*$/.test(value)) throw new Error("unsupported indentation: " + JSON.stringify("" + value));
         return value;
     }