test: check util.inspect for array having empty string as key

Refs: nodejs#15159

Author: mrahul17

test/parallel/test-util-inspect.js

@@ -968,6 +968,12 @@ if (typeof Symbol !== 'undefined') {
   assert.strictEqual(util.inspect(x), '{}');
 }
 
+{
+  const x = [];
+  x[''] = 1;
+  assert.strictEqual(util.inspect(x), '[ \'\': 1 ]');
+}
+
 // The following maxArrayLength tests were introduced after v6.0.0 was released.
 // Do not backport to v5/v4 unless all of
 // https://github.com/nodejs/node/pull/6334 is backported.