Accept null in markdown_to_html input

[janklan]

This PR allows passing null value to the markdown converter, returning an empty string instead of throwing an error.

Every significant Doctrine entity in my app has an optional Description field. I always push that value through the Markdown filter, and so I find myself always writing {% if entity.description %}{{ entity.description|markdown_to_html }}{% endif %}

The only reason for doing so is the fact markdown_to_html throws an error when receiving a null value. I think returning an empty value instead of a fatal error is a sane(r) default behaviour, hence this PR.

Thanks for reading.

[github-actions]

Thanks for your pull request! We love contributions.

However, you should instead open a pull request on the main repository:

https://github.com/twigphp/Twig

This repository is what we call a "subtree split": a read-only subset of that main repository.

We're looking forward to your PR there!