It seems not currently explicitly covered by https://www.mediawiki.org/wiki/Manual:Coding_conventions/PHP, but implicitly by example, at https://www.mediawiki.org/wiki/Manual:Coding_conventions/PHP#isset()
Do not use isset() to test for null. Using isset() in this situation could introduce errors by hiding misspelled variable names. Instead, use $var === null.
However, I've seen people point out in code review about prefering === null over is_null(). Assuming that believe is shared, we should consider enforcing and auto-fixing with PHCPS.