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Fix a marshalling error for columns of type `Maybe (Vector a)` #587

Job	Run time
continuous-integration (8.10.7)	2m 6s
continuous-integration (9.0.2)	2m 9s
continuous-integration (9.2.8)	1m 43s
continuous-integration (9.4.5)	2m 32s
continuous-integration (9.6.4)	2m 40s
continuous-integration (9.8.2)	3m 12s
continuous-integration (9.10.1)	1m 40s
continuous-integration (9.12.1, --allow-newer=base --allow-newer=template-haskell --allow-newer=g...	3m 26s
	19m 28s

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