Readonly property can be not set in if-else condition, and PHPStan yields no error

# Bug report

Readonly property can be not set in if-else condition, and PHPStan yields no error.

### Code snippet that reproduces the problem

https://phpstan.org/r/9d088cab-7c46-4d83-a75b-739900979c5b

### Expected output

This snippet should yield a "Readonly property not set in the constructor" issue.
