fix phpcs

Author: alcaeus

tests/UnifiedSpecTests/EventObserver.php

@@ -18,13 +18,13 @@
 use function assertInstanceOf;
 use function assertInternalType;
 use function assertNotEmpty;
-use function assertNotNull;
 use function assertObjectHasAttribute;
 use function assertSame;
 use function assertThat;
 use function count;
 use function current;
 use function get_class;
+use function is_object;
 use function key;
 use function MongoDB\Driver\Monitoring\addSubscriber;
 use function MongoDB\Driver\Monitoring\removeSubscriber;