sapyb to return even if out is not None

[paskino]

Changes in this pull request

With CIL https://github.com/TomographicImaging/CIL/pull/1742 sapyb needs to return even if the output is pre-allocated

Testing performed

Reintroduced test_SIRF.py from CIL. https://github.com/SyneRBI/SIRF-SuperBuild/pull/897

Related issues

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• [x] I have performed a self-review of my code
• [x] I have added docstrings/doxygen in line with the guidance in the developer guide
• [x] I have implemented unit tests that cover any new or modified functionality
• [ ] The code builds and runs on my machine
• [x] CHANGES.md has been updated with any functionality change

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