Offset access type where array has only one key fails with bound type string not supported

# Bug report

### Code snippet that reproduces the problem

It appears if the type T has only one key, T[K] fails because K is just a string and perhaps it expects always a union type of strings/keys?

https://phpstan.org/r/f4197fdb-eaef-485c-a5be-d026adf5e6c2

### Expected output

No error

cc @rvanvelzen 