fix: Cannot read properties of null (reading length) in stream-shift

[PatrickHeneise]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• [x] Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
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Fixes #2368

• https://github.com/mafintosh/stream-shift/commit/a5636fe3b835e58049e86b36dc5b7824e16e82ee
• https://github.com/mafintosh/duplexify/pull/40

[ddelgrosso1]

Thanks for updating this @PatrickHeneise