Array keys are not always set

# Bug report
When doing an isset before setting a key in an array phpstan will think the key is always set, but only if a 'child' array is set within that

### Code snippet that reproduces the problem

https://phpstan.org/r/f66b9fc9-d92f-4a7e-aa1b-34483e6cf92c

### Expected output

I'd expect both functions to not have any errors, but the first function has a failure.
