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": []
}
```

ext/json/generator/generator.c

@@ -681,6 +681,13 @@ static void generate_json_object(FBuffer *buffer, VALUE Vstate, JSON_Generator_S
     if (max_nesting != 0 && depth > max_nesting) {
         rb_raise(eNestingError, "nesting of %ld is too deep", --state->depth);
     }
+
+    if (RHASH_SIZE(obj) == 0) {
+        fbuffer_append(buffer, "{}", 2);
+        --state->depth;
+        return;
+    }
+
     fbuffer_append_char(buffer, '{');
 
     arg.buffer = buffer;
@@ -709,6 +716,13 @@ static void generate_json_array(FBuffer *buffer, VALUE Vstate, JSON_Generator_St
     if (max_nesting != 0 && depth > max_nesting) {
         rb_raise(eNestingError, "nesting of %ld is too deep", --state->depth);
     }
+
+    if (RARRAY_LEN(obj) == 0) {
+        fbuffer_append(buffer, "[]", 2);
+        --state->depth;
+        return;
+    }
+
     fbuffer_append_char(buffer, '[');
     if (RB_UNLIKELY(state->array_nl)) fbuffer_append(buffer, state->array_nl, state->array_nl_len);
     for(i = 0; i < RARRAY_LEN(obj); i++) {

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)