Various refactorings involving NULL, ASSERT, TRUE and FALSE

Various refactorings involving NULL, ASSERT, TRUE and FALSE

* Many more `NULL` -> `nullptr`
* More `ASSERT (ptr)` -> `ASSERT (ptr!=nullptr`)
* More `if (ptr)` -> `if (ptr != nullptr`)
* Also `if (!ptr)` -> `if (ptr == nullptr)`
* Also, treat `HWND` like `ptr` for these refactorings

* More `TRUE/FALSE` -> `true/false` where `bool` vars already exists
* All `ASSERT(FALSE)` are now `ASSERT(false)`
* Some `if ( i1 & i2 )` -> `if ( (i1 & i2) != 0)`
* Some `b == false` -> `!b`
* Some `b == true` -> `b`