Fixed warning when browsing Shared folder

The virtual "Shared" folder doesn't have an unencrypted_size field.
This fix adds a check to prevent warnings in the log.

Backport of 022d76c from master

apps/files_encryption/lib/proxy.php

@@ -343,7 +343,7 @@ class Proxy extends \OC_FileProxy {
 			\OC_FileProxy::$enabled = false;
 			$fileInfo = $view->getFileInfo($path);
 			\OC_FileProxy::$enabled = $proxyState;
-			if ($fileInfo['unencrypted_size'] > 0) {
+			if (isset($fileInfo['unencrypted_size']) && $fileInfo['unencrypted_size'] > 0) {
 				return $fileInfo['unencrypted_size'];
 			}
 			return $size;