Refactor assertions in AbstractAggregatorTest

The conditional checks in the AbstractAggregatorTest have been simplified. Instead of a complex inline condition with multiple checks, the test now uses straightforward assertions. This makes the code cleaner and easier to understand.

src/Bundle/ChillMainBundle/Test/Export/AbstractAggregatorTest.php

@@ -338,15 +338,11 @@ abstract class AbstractAggregatorTest extends KernelTestCase
                     .'is a string or an be converted to a string', $key)
             );
 
-            $this->assertTrue(
-                // conditions
-                \is_string((string) \call_user_func($closure, '_header'))
-                && !empty(\call_user_func($closure, '_header'))
-                && '_header' !== \call_user_func($closure, '_header'),
-                // message
-                sprintf('Test that the callable return by `getLabels` for key %s '
-                .'can provide an header', $key)
-            );
+            $head = \call_user_func($closure, '_header');
+
+            self::assertIsString($head);
+            self::assertNotEquals('', $head);
+            self::assertNotEquals('_header', $head);
         }
     }