tests: remove falsy empty string

This should not be here since `fs.mkdtemp()`/`fs.mkdtempSync()` also accepts an empty string in validation.

test/parallel/test-fs-mkdtemp-prefix-check.js

@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const fs = require('fs');
 
-const prefixValues = [undefined, null, 0, true, false, 1, ''];
+const prefixValues = [undefined, null, 0, true, false, 1];
 
 function fail(value) {
   assert.throws(