Fix PHPStan crashing with single value enum #304

Job	Run time
Platform test (8.1)	2m 43s
Platform test (7.4)	2m 33s
Platform test (8.0)	2m 42s
Platform test (8.4)	2m 33s
Platform test (8.3)	2m 34s
Platform test (8.2)	2m 51s
Platform test (8.2, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 29s
Platform test (8.4, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 35s
Platform test (8.3, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 34s
Platform test (8.1, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 33s
	26m 7s

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