Add `IsNull` property to `PWSTR` types.

[mjr4077au]

Is your feature request related to a problem? Please describe.

Currently if I need to check if the underlying value is null, I need to use an unsafe block to access the raw pointer, or I need to use an extension method to get me an IntPtr and then check if it's != IntPtr.Zero.

Describe the solution you'd like

Add an .IsNull property like most other pointer wrappers have.

Describe alternatives you've considered

My own extension property or method.

Additional context

N/A