dont draw box around DM subsystems if they are empty #692

Job	Run time
lint/style-and-typos	40s
linux/3.12	17m 37s
linux/3.8/bare-env	10s
osx/3.10	23m 59s
win/3.9	44m 26s
linux/3.11/conda	20m 2s
linux/3.10/mpi	8m 12s
linux/3.10/cython	21m 0s
win/3.8/pip	29m 32s
process-coverage-${{ matrix.TARGET }}	0s
	2h 45m 38s

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