Updating the title to actually reflect the issue - Zackery, could you update your PR and NEWS entry?

I just spent way too long looking for a MoveFileEx call that doesn't exist, because this isn't actually about doing things on reboot :)

For tests, you should be able to create your own REG_MULTI_SZ key with zero-length strings and read it back. If the winreg module won't let you do it, you may need ctypes (but that's okay in Windows-only tests).

Alternatively, if you want to dive deeper into the C API code, you could expose the fixupMultiSz function as a private function (winreg._split_multi_sz) and test it directly. That's my preferred way of unit testing things like this, but it's a decent amount of boilerplate just for a test.