Fix when_all() implementation under MSVC.
Fix when_all() implementation under MSVC.

I was relying on coroutine_handle() default constructor initialising
the handle to nullptr in when_all_awaitable_counter constructor.
But this is not the case with the current MSVC implementation.
Now explicitly initialise handle to nullptr.

lewissbaker/cppcoro,lewissbaker/cppcoro,lewissbaker/cppcoro,lewissbaker/cppcoro