fix(tests): Fix deprecation errors in S3 tests

Signed-off-by: Ferdinand Thiessen <[email protected]>

tests/lib/Files/ObjectStore/S3Test.php

@@ -106,6 +106,7 @@ public function testSeek() {
 	}
 
 	public function assertNoUpload($objectUrn) {
+		/** @var \OC\Files\ObjectStore\S3 */
 		$s3 = $this->getInstance();
 		$s3client = $s3->getConnection();
 		$uploads = $s3client->listMultipartUploads([
@@ -119,7 +120,7 @@ public function testEmptyUpload() {
 		$s3 = $this->getInstance();
 
 		$emptyStream = fopen("php://memory", "r");
-		fwrite($emptyStream, null);
+		fwrite($emptyStream, '');
 
 		$s3->writeObject('emptystream', $emptyStream);