NULL and null as a property default value are treated differently when overriding a parent property

[mabar]

Bug report

Parent property supports only string. When default value is overriden in child, NULL is correctly reported but null is not.

Code snippet that reproduces the problem

https://phpstan.org/r/b449bcf7-6aba-4362-bb63-7ce1c6d9d0e2

Expected output

Both cases should report an error

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