Convert isset ternary to null coalescing operator

Signed-off-by: Hamid Dehnavi <hamid.dev.pro@gmail.com>

tests/lib/Util/User/Dummy.php

@@ -168,7 +168,7 @@ class Dummy extends Backend implements \OCP\IUserBackend {
 	}
 
 	public function getDisplayName($uid) {
-		return isset($this->displayNames[$uid])? $this->displayNames[$uid]: $uid;
+		return $this->displayNames[$uid] ?? $uid;
 	}
 
 	/**

tests/lib/UtilCheckServerTest.php

@@ -30,7 +30,7 @@ class UtilCheckServerTest extends \Test\TestCase {
 		$config->expects($this->any())
 			->method('getValue')
 			->willReturnCallback(function ($key, $default) use ($systemOptions) {
-				return isset($systemOptions[$key]) ? $systemOptions[$key] : $default;
+				return $systemOptions[$key] ?? $default;
 			});
 		return $config;
 	}