[C++][pistache-server] Optional integer query parameter is not supported

[yvesdm3000]

PR checklist

• [OK] Read the contribution guidelines.
• [OK] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [OK] Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• [OK] Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

The generated pistache code doesn't support an optional query parameter that is declared as being an integer. This code in the mustache templates fixes this issue.