The Windows, Linux, and GLFW embeddings (which all share a common code
ancestry) pass TextInput.setEditingState selection base and extents
straight through to the shared text model class. The model expects those
values to be valid, but the framework sends -1/-1 for "invalid"
selections, which happen for some empty text cases (e.g.,
TextFieldController.clear()).
This translates those invalid selection values to an empty selection at
the start of the string, as expected by the model.
Fixes https://github.com/flutter/flutter/issues/59140
