Change accessibility notifier creation so we do not create one when we're in PTY mode. (Guard all call sites to skip math/event work when the notifier is null.) MSAA events are legacy events that are registered for globally and used by some screen readers to find content in the conhost window. The PTY mode is not responsible for hosting the display content or input window, so it makes sense for it to not broadcast these events and delegate the accessibility requirement to the connected terminal.
## References
- #10537
## PR Checklist
* [x] Closes#10568
* [x] I work here
* [x] Manual test launches passed.
91b454ac95