Generic/DisallowYodaConditions: ?? should not trigger the sniff

[rodrigoprimo]

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

Types of changes

• 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

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• 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).
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added XML documentation for the sniff.

[jrfnl]

Good call @rodrigoprimo. LGTM.

For the record: it is unlikely that the null coalesce operator being listened to by this sniff would ever have caused an issue in real life code as the code sample - as per the test file - which would trigger the sniff, is not a realistic code sample.

1 ?? $var desugars to isset(1) ? 1 : $var and doing an isset() on a hard-coded value doesn't make any sense.