[BUG][C++-rest-sdk] Issue when default value is nullptr

Created by: dan-drl

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Run time error occurs because default type is set to nullptr. This breaks the fromJSON and toJSON methods in model-source.mustache.

openapi-generator version

4.1.0-SNAPSHOT

OpenAPI declaration file content or url

See test.json: https://gist.github.com/dan-drl/147e962298d1f1467638c00786cd9b6f See resulting c++: https://gist.github.com/dan-drl/0ec11853c939d5d05ab3c2699238d324

Command line used for generation

java -jar openapi-generator-cli.jar generate -i test.json -g cpp-restsdk -o \src\test

Steps to reproduce

If a JSON containing a "value":null is provided to the AdditionalField model for deserialization, a runtime error occurs because of this:

std::shared_ptr<Object> newValue(nullptr);
newValue->fromJson(val.at(utility::conversions::to_string_t(L"value")));

Related issues/PRs

I didn't find anything.

Suggest a fix

Planning on making a pull request. See this commit, https://github.com/dan-drl/openapi-generator/commit/7b42948498ab0a82ee25c875a9787cdc685c9890