Imploding the keys of a filtered array is considered true

# Bug report

I have a method where the keys of a filtered array is imploded while this can be an empty string phpstan always considers it true.

### Code snippet that reproduces the problem

https://phpstan.org/r/9c71b566-6dd5-4c44-b0e2-36ba7db46b42

### Expected output

The first two chunk of code to produce errors, the last two to pass.