Fix numba implementation of empty DimShuffle

[aseyboldt]

The previous version returned a scalar instead of an array, which can then lead to errors in later code that expects to receive an array.

Fixes pymc-devs/nutpie#37

Checklist

• Explain motivation and implementation 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues, preferably in nice commit messages.
• The commits correspond to relevant logical changes. Note that if they don't, we will rewrite/rebase/squash the git history before merging.
• Are the changes covered by tests and docstrings?

[ricardoV94]

You have a float32 downcast issue in your test, otherwise looks good

[ricardoV94]

just so we don't merge accidentally

[ricardoV94]

I updated protection settings to require tests to pass before merging