Delete nullptr Json::Value constructor (#1194)

This patch adds an explicit ctor with a std::nullptr_t argument, that is `delete`-d. This keeps Json::Value from exposing a coding error when automatically promoted to a const char* type.

include/json/value.h

@@ -342,6 +342,7 @@ public:
   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();