Nested captured optional subpatterns result in non-falsy-string instead of string

### Bug report

When nesting capturing subpatterns that have both empty and non-empty branches, phpstan assumes the resulting type is non-falsy-string, instead of string.

### Code snippet that reproduces the problem

https://phpstan.org/r/8c787e73-b367-4dfb-9a6f-0c5eacae72a2

### Expected output

"No errors!"

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