nullCoalesce.offset and isset.offset report an error on an array offset that can be null #13488

Bug report

Follow up on #11708 which didnt solve all the cases in the comments.

Code snippet that reproduces the problem

https://phpstan.org/r/2b9a0f2b-978d-4125-a91d-acb32df3f99e

Expected output

PHPStan should not memorize $randomString to be set after using ?? null. The isset() checks are wrongly reprted as unnecessary as revealed in this snippet: https://3v4l.org/nhWm3

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