Description

This PR changes the Generic.ControlStructures.DisallowYodaConditions sniff so that the null coalescing operator (??) does not trigger it.

Suggested changelog entry

Generic.ControlStructures.DisallowYodaConditions: null coalescing operator (??) should not trigger the sniff

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
- [ ] This change is only breaking for integrators, not for external standards or end-users.
[ ] Documentation improvement

[x] I have checked there is no other PR open for the same change.
[x] I have read the Contribution Guidelines.
[x] I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
[x] I have added tests to cover my changes.
[x] I have verified that the code complies with the projects coding standards.
[ ] [Required for new sniffs] I have added XML documentation for the sniff.