Correctly handle Redis::keys returning false

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

build/stubs/redis.php

      *
      * @param string $pattern pattern, using '*' as a wildcard
      *
-     * @return array string[] The keys that match a certain pattern.
+     * @return string[]|false The keys that match a certain pattern.
      *
      * @link    https://redis.io/commands/keys
      * @example

lib/private/Memcache/Redis.php

 		$keys = self::$cache->keys($prefix);
 		$deleted = self::$cache->del($keys);
 
-		return count($keys) === $deleted;
+		return (is_array($keys) && (count($keys) === $deleted));
 	}
 
 	/**