lib: fix TypeError in v8-polyfill

PR-URL: https://github.com/nodejs/node/pull/8863
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/internal/v8_prof_polyfill.js

@@ -76,7 +76,7 @@ function readline() {
     const bytes = fs.readSync(fd, buf, 0, buf.length);
     line += dec.write(buf.slice(0, bytes));
     if (line.length === 0) {
-      return false;
+      return '';
     }
   }
 }