[NodeAnalyzer] Handle no repositoryClass on RepositoryClassResolver (#379)

avoid error:

```
         "System error: "PHPStan\Reflection\ReflectionProvider\MemoizingReflectionProvider::hasClass(): Argument #1     
         ($className) must be of type string, null given, called in   
```

Author: samsonasik

src/NodeAnalyzer/RepositoryClassResolver.php

@@ -56,6 +56,10 @@ public function resolveFromEntityClass(string $entityClassName): ?string
             $repositoryClass = $match['repositoryClass'];
         }
 
+        if ($repositoryClass === null) {
+            return null;
+        }
+
         if (! $this->reflectionProvider->hasClass($repositoryClass)) {
             throw new ShouldNotHappenException(
                 sprintf('Repository class "%s" for entity "%s" does not exist', $repositoryClass, $entityClassName)