Fix false positive with array_combine() on php8

[staabm]

closes phpstan/phpstan#9011

test fails without fix:

There was 1 failure:

1) PHPStan\Rules\Methods\ReturnTypeRuleTest::testBug9011
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'
+'16: Method Bug9011\HelloWorld::getX() should return array<string> but returns false.
 '

C:\dvl\Workspace\phpstan-src-staabm\src\Testing\RuleTestCase.php:154
C:\dvl\Workspace\phpstan-src-staabm\tests\PHPStan\Rules\Methods\ReturnTypeRuleTest.php:851

---

the 2nd example posted in the issue works as expected in my eyes.

[phpstan-bot]

This pull request has been marked as ready for review.

[phpstan-bot]

This pull request has been marked as ready for review.

[ondrejmirtes]

Thank you.