Unity C#: Change speed when all Targets are destroyed

Question

I have a game where a Gunner moves on a pre-set path towards a final destination. You play as a Magic Bullet that auto-fires from the Gunner's current position and can be controlled with WASD. When the bullet collides with a Target, it's destroyed.

My question: How do I change the speed of the Gunner after all of the Targets are destroyed? The idea if for the Gunner to hurry to the end, instead of skilled players waiting around.

Any help would be greatly appreciated. Here's what I have so far:

using UnityEngine;
using System.Collections;

public class MovementPathScript : MonoBehaviour {

    public GunnerPathScript PathToFollow; 
    public int CurrentWayPointID = 0;

    private float speedVariable;
    public float gunnerSpeed;
    public float hurrySpeed; 

    private float reachDistance = 1.0f;
    public float rotatationSpeed = 5.0f;

    public string pathName;

    public static Quaternion gunnerRotation;

    private Vector3 last_position;
    private Vector3 current_position; 

    void Start () {
        last_position = transform.position; 
    }

    void Update () {

        float distance = Vector3.Distance (PathToFollow.path_objs [CurrentWayPointID].position, transform.position); 
        GunnerMove ();

        PointTowards (); 

        if (distance <= reachDistance) {
            CurrentWayPointID++;
        }

        if (CurrentWayPointID >= PathToFollow.path_objs.Count) {
            CurrentWayPointID = 13;
        }
    }

        void GunnerMove(){
            transform.position = Vector3.MoveTowards (transform.position, PathToFollow.path_objs [CurrentWayPointID].position, Time.deltaTime * speedVariable);  
            speedVariable = gunnerSpeed; 

        if (GameObject.FindGameObjectsWithTag ("Target") == null) {  
            speedVariable = hurrySpeed;  
            //Unity doesn't recognize this, and maintains gunnerSpeed

        }
    }

    void OnTriggerEnter(Collider other){
        if (other.gameObject.CompareTag ("Target"))
            Destroy (this.gameObject);
    }

    public void PointTowards(){ 
        var gunnerRotation = Quaternion.LookRotation (PathToFollow.path_objs [CurrentWayPointID].position - transform.position);
        transform.rotation = Quaternion.Lerp(transform.rotation, gunnerRotation, Time.deltaTime * rotatationSpeed); 
    }
}

Answer 1

Change this line:

if (GameObject.FindGameObjectsWithTag ("Target") == null) {  

To:

if (GameObject.FindGameObjectsWithTag ("Target").Length == 0) {

Reason: FindGameObjectsWithTag returns an array, it can never be null, but it can be "empty". So you need to check it's length.

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Also note that this is an inefficient way of target tracking. It would be better if you keep a list of targets somewhere and check the count of that list.