Fix PHPStan crashing with single value enum #305

Job	Run time
Platform test (8.0)	2m 54s
Platform test (8.2, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 27s
Platform test (7.4)	2m 37s
Platform test (8.3, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 48s
Platform test (8.2)	2m 39s
Platform test (8.1)	2m 40s
Platform test (8.4)	2m 32s
Platform test (8.3)	2m 47s
Platform test (8.4, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 29s
Platform test (8.1, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 29s
	26m 22s

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