Fix Stringified converters

[flobernd]

Fix Stringified converters to allow them being used on non-nullable types.

As, for example, Nullable<int> and int are distinct types, STJ refused to work with the StringifiedInt<int?> converter if used on a simple int property.

STJ does not invoke custom converters for null values unless the converter opts-in by returning true in the HandleNull method. As we don't do this, we don't have to care about null in the converter itself.

Handling e.g. just int instead of int? in the stringified converter, allows us to annotate int and int? properties correctly without having to introduce another converter type.

Closes #7870

[github-actions]

The backport to serverless failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-serverless serverless
# Navigate to the new working tree
cd .worktrees/backport-serverless
# Create a new branch
git switch --create backport-7965-to-serverless
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 fc76ed7f4703ed81cc846bd59e6338bbebf37e7e
# Push it to GitHub
git push --set-upstream origin backport-7965-to-serverless
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-serverless

Then, create a pull request where the base branch is serverless and the compare/head branch is backport-7965-to-serverless.