fix(files_external): Be extra safe with null handling to please psalm

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

apps/files_external/lib/Lib/Storage/SFTP.php

 		$result = $this->getConnection()->put($this->absPath($path), $stream);
 		fclose($stream);
 		if ($result) {
+			if ($size === null) {
+				throw new \Exception("Failed to get written size from sftp storage wrapper");
+			}
 			return $size;
 		} else {
 			throw new \Exception("Failed to write steam to sftp storage");