isdb-SAXS: small update for a NA corner case #2924

Job	Run time
codecheck / codecheck	43m 47s
conda / conda (ubuntu-20.04)	10m 45s
conda / conda (macos-12)	16m 26s
docker / docker (fedora39)	19m 42s
conda / conda (macos-14)	8m 35s
docker / docker (rocky8)	23m 38s
linux / linux (-doc-mpi-)	39m 22s
linux / linux (-mpi-)	51m 30s
linux / linux (-coverage-mpi-)	1h 0m 57s
mac / macports (macos-12)	16m 31s
linux / linux (-debug-)	29m 0s
mac / macsimple (macos-12)	9m 46s
linux / linux (-debug-mpi-)	39m 23s
mac / macports (macos-14)	9m 38s
mac / macsimple (macos-14)	4m 30s
linux / linux (-intel-)	44m 30s
linux / linux (-pycv-mpi-)	16m 46s
mac / macports (+allmodules, macos-12)	20m 16s
	7h 45m 2s

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