Cythonize is_null_slice
This makes the diff in #54506 smaller
Is there a reason you want to Cythonize this? (impact on benchmarks?)

This seems to be used only 6 times.
https://github.com/search?q=repo%3Apandas-dev%2Fpandas%20is_null_slice&type=code Yea it also would get used in https://github.com/pandas-dev/pandas/pull/54506 in the getitem / setitem methods of the BitmaskArray I won't claim that this has a significant impact on benchmarks. The main thing is that it shows less Python interaction in Cython annotate, so it causes less visual noise when searching through the annotate to find bottlenecks. If there's unease about this then no problem closing