"Comparison to null is expensive" warning can be misleading

The warning, [Avoid null comparisons against UnityEngine.Object subclasses](https://github.com/JetBrains/resharper-unity/wiki/Avoid-null-comparisons-against-UnityEngine.Object-subclasses) shows for this code:
```
if (obj != null) DoSomething();
```
But not showing for this code:
```
if (obj) DoSomething();
```
Since bool cast operator of `UnityEngine.Object` does the same thing as comparing to `null`, the warning should be shown as same. Otherwise, it can mislead users to use bool cast instead, which is worse practice as it is less explicit.