fix: check if defined WRITEPATH exists

[michalsn]

Description
This PR adds a check that WRITEPATH is defined correctly.

Fixes: #9309

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[neznaika0]

Do I need to use DS? Windows is not supported

[michalsn]

@neznaika0 realpath() handles directory separators automatically. You can use / in the code, even on Windows, as PHP converts it to \ where necessary.

[neznaika0]

I know that the paths are right for the system. But for tests and other manipulations, this is a problem - different slashes

[michalsn]

Thanks everyone!