[Finder] Fix using == as default operator in DateComparator

Author: MatTheCat

src/Symfony/Component/Finder/Comparator/DateComparator.php

@@ -36,7 +36,7 @@ public function __construct(string $test)
             throw new \InvalidArgumentException(sprintf('"%s" is not a valid date.', $matches[2]));
         }
 
-        $operator = $matches[1] ?? '==';
+        $operator = $matches[1] ?: '==';
         if ('since' === $operator || 'after' === $operator) {
             $operator = '>';
         }

src/Symfony/Component/Finder/Tests/Comparator/DateComparatorTest.php

@@ -59,6 +59,7 @@ public static function getTestData()
             ['after 2005-10-10', [strtotime('2005-10-15')], [strtotime('2005-10-09')]],
             ['since 2005-10-10', [strtotime('2005-10-15')], [strtotime('2005-10-09')]],
             ['!= 2005-10-10', [strtotime('2005-10-11')], [strtotime('2005-10-10')]],
+            ['2005-10-10', [strtotime('2005-10-10')], [strtotime('2005-10-11')]],
         ];
     }
 }