Check if a GameObject has been assigned in the inspector in Unity3D 2019.3.05f

Unity has a custom implementation of equality == operator (see Custom == operator, should we keep it? which often leads to confusion / unexpected behavior.

Even if an UnityEngine.Object has a value equal to null it is sometimes not == null.
Object rather still stores some meta data in it e.g. you get a MissingReferenceException, not a usual NullReferenceException because Unity implemented some custom exceptions that often hold more detail to why the value is equal to null.

Especially since

public GameObject testObject = null;

is a public field it is automatically serialized and thus the null value you assigned to it will be overwritten anyway during the serialization.

---

UnityEngine.Object which GameObject derives from has an implicit operator bool

Does the object exist?

Instead of a direct == null comparison you should rather check for

public static bool IsNull<T>(this T myObject, string message = "") where T : UnityEngine.Object
{
    if(!myObject)
    {
        Debug.LogError("The object is null! " + message);
        return false;
    }

    return true;
}

---

For a general method working for any reference type you could e.g. use something like

public static bool IsNull<T>(this T myObject, string message = "") where T : class
{
    if (myObject is UnityEngine.Object obj)
    {
        if (!obj)
        {
            Debug.LogError("The object is null! " + message);
            return false;
        }
    }
    else
    {
        if (myObject == null)
        {
            Debug.LogError("The object is null! " + message);
            return false;
        }
    }

    return true;
}