Fix latest phpstan assert

Author: samsonasik

rules/DeadCode/Rector/FunctionLike/NarrowWideUnionReturnTypeRector.php

@@ -126,8 +126,8 @@ public function refactor(Node $node): ?Node
         $hasImplicitNullReturn = $this->silentVoidResolver->hasSilentVoid($node)
             || $this->hasImplicitNullReturn($returnStatements);
 
+        /** @var UnionType|NullableType $returnType */
         $returnType = $node->returnType;
-        Assert::isInstanceOfAny($returnType, [UnionType::class, NullableType::class]);
 
         $returnType = $this->staticTypeMapper->mapPhpParserNodePHPStanType($returnType);
         $actualReturnTypes = $this->collectActualReturnTypes($returnStatements);