Avoiding errors due to an empty array

Let's make our code a little more robust so that it can cope with the issue of an empty spawnPoints array; that is, when no objects are tagged as Respawn in the scene.

To cope with no objects being tagged as Respawn, we need to do the following:

1. Improve our Start() method in the C# script class called SpawnPointManager so that an error is logged if the array of the objects tagged as Respawn is empty:

  void Start() {
       spawnPoints = GameObject.FindGameObjectsWithTag("Respawn");

       // logError if array empty
       if(spawnPoints.Length < 1)
         Debug.LogError ("SpawnPointManagaer - cannot find any objects 
         tagged 'Respawn'!");
     }

2. Improve the RandomSpawnPoint() and NearestSpawnpoint() methods in the C# script class called SpawnPointManager so that they still return a value (that is, null), even if the array is empty:

    public GameObject RandomSpawnPoint ...