Ensure called `default_sort_func` is not None #1140

Job	Run time
test-windows (3.9, numpy>=1.21,<2.0.0)	1m 1s
test-linux (3.9)	1m 54s
test-windows (3.10, numpy>=1.21,<2.0.0)	57s
test-linux (3.9, uncertainties==3.1.6)	1m 44s
test-macos (3.9)	3m 1s
test-linux (3.9, uncertainties>=3.1.6,<4.0.0)	1m 45s
test-windows (3.11, numpy>=1.21,<2.0.0)	1m 5s
test-macos (3.9, numpy>=1.21,<2.0.0)	3m 36s
test-linux (3.9, numpy>=1.21,<2.0.0)	1m 50s
test-linux (3.9, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	2m 24s
test-macos (3.10)	2m 0s
test-linux (3.9, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	2m 21s
test-linux (3.10)	1m 43s
test-macos (3.10, numpy>=1.21,<2.0.0)	2m 6s
test-linux (3.10, uncertainties==3.1.6)	1m 41s
test-macos (3.11)	3m 5s
test-linux (3.10, uncertainties>=3.1.6,<4.0.0)	1m 45s
test-macos (3.11, numpy>=1.21,<2.0.0)	3m 11s
test-linux (3.10, numpy>=1.21,<2.0.0)	1m 50s
test-linux (3.10, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	2m 32s
test-linux (3.10, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	2m 28s
test-linux (3.11)	1m 47s
test-linux (3.11, uncertainties==3.1.6)	1m 41s
test-linux (3.11, uncertainties>=3.1.6,<4.0.0)	1m 43s
test-linux (3.11, numpy>=1.21,<2.0.0)	1m 39s
test-linux (3.11, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	2m 16s
test-linux (3.11, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	2m 19s
test-linux (3.9, numpy, matplotlib==2.2.5)	3m 26s
test-linux (3.9, numpy, uncertainties, sparse xarray netCDF4 dask[complete]==2023.4.0 graphviz ba...	3m 49s
	1h 2m 39s

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