FIX Treat value not in SingleSelectField options as blank #2098

Job	Run time
CI / Generate matrix	15s
CI / Context	1s
CI / Check governance	1s
CI / 8.1 prf-low mysql57 phpunit framework-core	5m 17s
CI / 8.2 mariadb phpunit framework-core	5m 14s
CI / 8.3 mysql80 phpunit framework-core	7m 11s
CI / 8.3 mysql84 phpunit framework-core	7m 12s
CI / 8.1 prf-low mysql57 phpunit framework-orm	10m 41s
CI / 8.2 mariadb phpunit framework-orm	10m 43s
CI / 8.3 mysql80 phpunit framework-orm	11m 27s
CI / 8.3 mysql84 phpunit framework-orm	11m 43s
CI / 8.1 prf-low mysql57 phpunit cms	1m 36s
CI / 8.2 mariadb phpunit cms	2m 3s
CI / 8.3 mysql80 phpunit cms	1m 42s
CI / 8.3 mysql84 phpunit cms	1m 58s
CI / 8.1 mysql57 phplinting	2m 57s
CI / Dispatch tag patch release	0s
	1h 20m 1s

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