do not fail with default config

Author: freekmurze

src/Helpers/ConfigurationValidator.php

@@ -28,6 +28,14 @@ public static function validate(?array $feeds = null): void
 
     protected static function validateFeedItemsResolver($resolver): bool
     {
+        if ($resolver === null) {
+            return true;
+        }
+
+        if ($resolver === '') {
+            return true;
+        }
+
         if (! is_string($resolver) && ! is_array($resolver)) {
             return false;
         }

tests/ConfigurationValidatorTest.php

@@ -50,7 +50,7 @@ public function it_throws_an_exception_for_an_invalid_items_value()
     {
         $exceptionCounter = 0;
 
-        $invalidItems = ['', null, [], ['test']];
+        $invalidItems = [[], ['test']];
         $validItems = ['Model@getAll', ['App\\Model', 'getItems'], ['App\\Model', 'getItems', 'param1']];
 
         $items = array_merge($invalidItems, $validItems);