Add workaround for scalar document is_null bug

For now, we can just manually throw the exception until the next
simdjson bugfix release: simdjson/simdjson#2258

src/luasimdjson.cpp

@@ -124,6 +124,9 @@ void convert_ondemand_element_to_table(lua_State *L, T& element) {
       // calling is_null().value() will trigger an exception if the value is invalid
       if (element.is_null().value()) {
         lua_pushlightuserdata(L, NULL);
+      } else {
+        // workaround for simdjson 3.10.1
+        throw simdjson_error(INCORRECT_TYPE);
       }
       break;
   }