When serializing with JSON_C_TO_STRING_PRETTY set, keep the opening and closing curly or square braces on same line for empty objects or arrays. Issue #778.

tests/test1Formatted_pretty.expected

@@ -97,5 +97,7 @@ my_object.to_string()={
   "abc":12,
   "foo":"bar",
   "bool0":false,
-  "bool1":true
+  "bool1":true,
+  "empty_array":[],
+  "empty_obj":{}
 }