Fixes https://github.com/flutter/flutter/issues/79495

This is basically a reland of https://github.com/flutter/flutter/pull/79607.

Currently when the cursor is invalid, arrow key navigation / typing / backspacing doesn't work since the cursor position is unknown. 
Showing the cursor when the selection is invalid gives the user the wrong information about the current insert point in the text. 

This is going to break internal golden tests.