DM-54947: Fix assembleCellCoadd handling of fully-masked cells #705

Job	Run time
call-workflow / check-tooling	12s
call-workflow / lint_with_ruff	6s
call-workflow / format_with_black	9s
call-workflow / lint_with_flake8	0s
call-workflow / format_with_ruff	0s
call-workflow / isort	0s
	27s

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