fix #3646, forcefully change unsupported input to strings (#3658)

Co-authored-by: gzb1128 <gzb1128@foxmail.com>

Author: gzb1128

packages/less/Gruntfile.js

@@ -261,6 +261,7 @@ module.exports = function(grunt) {
                     `node bin/lessc --clean-css="--s1 --advanced" ${lessFolder}/_main/lazy-eval.less tmp/lazy-eval.css`,
                     "cd lib",
                     `node ../bin/lessc --clean-css="--s1 --advanced" ../${lessFolder}/_main/lazy-eval.less ../tmp/lazy-eval.css`,
+                    `node ../bin/lessc --source-map=lazy-eval.css.map --autoprefix ../${lessFolder}/_main/lazy-eval.less ../tmp/lazy-eval.css`,
                     "cd ..",
                     // Test multiple plugins
                     `node bin/lessc --plugin=clean-css="--s1 --advanced" --plugin=autoprefix="ie 11,Edge >= 13,Chrome >= 47,Firefox >= 45,iOS >= 9.2,Safari >= 9" ${lessFolder}/_main/lazy-eval.less tmp/lazy-eval.css`

packages/less/bin/lessc

@@ -180,6 +180,9 @@ function render() {
       var filename = sourceMapOptions.sourceMapFullFilename;
       ensureDirectory(filename);
 
+      //To fix https://github.com/less/less.js/issues/3646
+      output=output.toString();
+      
       fs.writeFile(filename, output, 'utf8', function (err) {
         if (err) {
           var description = 'Error: ';