Ampache: Set the has_art property properly in the user action

On recent versions of Nextcloud, the avatar is always available; either
as user-provided or as a placeholder graphic. On ownCloud, on the other
hand, the avatar is available via the normal URL only if it has been
provided by the user. Otherwise, the URL returns a simple JSON content.
In those cases, the `has_art` is now returned as false.

lib/Controller/AmpacheController.php

@@ -1411,7 +1411,7 @@ protected function user(?string $username) : array {
 			'state' => null,
 			'city' => null,
 			'art' => $this->urlGenerator->linkToRouteAbsolute('core.avatar.getAvatar', ['userId' => $user->getUID(), 'size' => 64]),
-			'has_art' => true
+			'has_art' => ($user->getAvatarImage(64) != null)
 		];
 	}