fix: don't needlessly contrain join when the left side is allow to be null

[icewind1991]

Summary

Currently, the left side of a LEFT JOIN is constrained, we implicitly transform it into an INNER JOIN. With the reasoning being that a row needs to exist to satisfy the constraint.
However, the constrain might still allow unset rows by allowing null values in the constraint. For example in: RuleManager::getRulesForFilesByParent

This changes it to only do the transform to INNER JOIN if the constraint doesn't allow null rows.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI