`non-falsy-string` includes an empty string sometimes

# Bug report

If '0' is removed from `string`, it becomes `non-falsy-string`.
`string` should become `non-falsy-string` only when '0' and '' were removed.

### Code snippet that reproduces the problem

https://phpstan.org/r/344914e6-93a6-4c7a-b39b-15d19f7765a9

### Expected output

Just `string` I guess.

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