Ampache: Return all results if argument limit=0 given

This is how the "real" Ampache server works, too, although this is not
documented in the current API specification for v4 or v5 API. The Amarok
player uses this argument value, and there should be no real-life case where
the client actually would want to have 0 interpreted as literal value of the
limit.

Furthermore, the Music app already tried to interpret zero as "unlimited" but
this worked only with integer 0 and not with string "0". Whether the numeric
arguments come as strings or integers, depends on the details of the hosting
cloud, and may differ between versions of ownCloud and Nextcloud.

refs #854

CHANGELOG.md

@@ -18,6 +18,8 @@
   [#849](https://github.com/owncloud/music/issues/849)
 - Subsonic method `getPlaylist` breaking if the list has any invalid tracks
   [#853](https://github.com/owncloud/music/issues/853)
+- Ampache methods returning empty result sets to Amarok which passes (invalid) argument `limit=0`
+  [#854](https://github.com/owncloud/music/issues/854)
 
 ## 1.1.0 - 2021-03-24
 ### Added

lib/Controller/AmpacheController.php

@@ -9,7 +9,7 @@
  * @author Morris Jobke <hey@morrisjobke.de>
  * @author Pauli Järvinen <pauli.jarvinen@gmail.com>
  * @copyright Morris Jobke 2013, 2014
- * @copyright Pauli Järvinen 2017 - 2020
+ * @copyright Pauli Järvinen 2017 - 2021
  */
 
 namespace OCA\Music\Controller;
@@ -881,11 +881,8 @@ private function createCoverUrl($entity, $auth) {
 	 * @return integer|null
 	 */
 	private static function validateLimitOrOffset($value) : ?int {
-		if (\ctype_digit(\strval($value)) && $value !== 0) {
-			return \intval($value);
-		} else {
-			return null;
-		}
+		$value = (int)$value;
+		return ($value > 0) ? $value : null;
 	}
 
 	/**