pretty_generate: don't apply object_nl / array_nl for empty containers

Fix: https://github.com/ruby/json/issues/437

Before:

```json
{
  "foo": {
  },
  "bar": [
  ]
}
```

After:

```json
{
  "foo": {},
  "bar": []
}
```

1 files changed, 7 insertions, 0 deletions

diff --git a/test/json/json_generator_test.rb b/test/json/json_generator_test.rb
index 7dc45e3a52..32712c8343 100755
--- a/test/json/json_generator_test.rb
+++ b/test/json/json_generator_test.rb
@@ -90,10 +90,17 @@ EOT
 
   def test_generate_pretty
     json = pretty_generate({})
+    assert_equal('{}', json)
+
+    json = pretty_generate({1=>{}, 2=>[], 3=>4})
     assert_equal(<<'EOT'.chomp, json)
 {
+  "1": {},
+  "2": [],
+  "3": 4
 }
 EOT
+
     json = pretty_generate(@hash)
     # hashes aren't (insertion) ordered on every ruby implementation
     # assert_equal(@json3, json)