Fixed always-true condition in world loading

this has a couple of side effects which need to be explored.
- first of all, this bug prevented generateLevel() from filling in the preset from server.properties. With this fix, worlds which don't have any extra generator settings will start to be generated using server.properties settings, which is almost certainly not expected behaviour.
- preset can now be specified separately from generator in pocketmine.yml, which is nicer for users.

src/pocketmine/Server.php

@@ -1543,7 +1543,7 @@ class Server{
 					if(isset($options["generator"])){
 						$generatorOptions = explode(":", $options["generator"]);
 						$generator = GeneratorManager::getGenerator(array_shift($generatorOptions));
-						if(count($options) > 0){
+						if(count($generatorOptions) > 0){
 							$options["preset"] = implode(":", $generatorOptions);
 						}
 					}else{