Remove unnecessary null pointer checks

[elfring]

An extra null pointer check is not needed in functions like the following.

• RawAsset<T>::UnloadInternal
• Scene::SetStarfield
• Singleton<T>::DestroyInstance

[codenamecpp]

i use this

template <typename TElement>
inline void SafeDelete(TElement*& elementPointer)
{
    if (elementPointer)
    {
        delete elementPointer;
        elementPointer = nullptr;
    }
}

[elfring]

@codenamecpp: I find that your example for a function template contains an unnecessary null pointer check.