merged branch Tobion/random-fix (PR #693)

Commits
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06361ca added note in changelog
2706412 fixed random function when charset is null

Discussion
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Fixed random function when charset is null

Fixed random function when charset is null and the value is an empty string, resulting in an mt_rand error max(-1) is smaller than min(0).
Also fixed a test on windows.

But I don't know why you introduced the special handling for `null === $env->getCharset()`.
Why is that necessary and why is it possible to have a null charset at all? Usually it defaults to utf-8 anyway.