Simplify polymorphic has_one handling from PR #32 #146

Job	Run time
ci / Generate matrix	13s
ci / Context	2s
ci / Check governance	2s
ci / 8.1 mysql57 phplinting	1m 54s
ci / 8.2 mariadb phpunit all	2m 2s
ci / 8.1 prf-low mysql57 phpunit all	2m 8s
ci / 8.3 mysql84 phpunit all	2m 30s
ci / 8.3 mysql80 phpunit all	2m 27s
ci / Dispatch tag patch release	0s
	11m 18s

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