Delete nullptr Json::Value constructor

Author: nathanruiz

include/json/value.h

@@ -342,6 +342,7 @@ class JSON_API Value {
   Value(const StaticString& value);
   Value(const String& value);
   Value(bool value);
+  Value(std::nullptr_t ptr) = delete;
   Value(const Value& other);
   Value(Value&& other);
   ~Value();