Redundant Condition Detection doesn't account for initial 'unset' state from `!isset($this->y)`

Code Snippet: https://3v4l.org/lEbfg (full code also provided at end of issue)
Phan version used: 5.4.1
Observed behavior:

test.php:10 PhanRedundantCondition Redundant attempt to cast $this->y of type string to isset

Expected behavior: No errors (in the real world case, then $this->y might or might not be set based on a more complex set of ifs in the constructor, and the property is readonly; so I can't set it at the top of the function and then set it to a different value later.

Full test code:

<?php
class y{
	public string $y;
	
	public function __construct(bool $b){
		if($b){
			$this->y = "hello";
		}
		
		if(!isset($this->y)){
			$this->y = "goodbye";
		}
	}
}

Reproduce the report using the full PHP example in the issue or the linked 3v4l.org snippet with Phan 5.4.1. Start by tracing the redundant-condition analysis that emits PhanRedundantCondition for the isset check; done means the valid initial unset state no longer produces that warning.