Change Model::row_data to return an Option<T> (#873)

Change Model::row_data to return an Option<T> (rust) or std::optional<T> (c++)

Co-authored-by: Olivier Goffart <olivier@woboq.com>
Co-authored-by: Simon Hausmann <hausmann@gmail.com>

api/sixtyfps-cpp/docs/cpp_migration.md

@@ -6,7 +6,7 @@ This guide lists all API incompatible changes between major versions and describ
 
 ## Migrating from Version 0.1.x to 0.2.0
 
-In the 0.2.x series we have increased the minimum version of C++. You need to have a C++ compiler installed that supports C++ 20 or newer.
+In version 0.2.0 we have increased the minimum version of C++. You need to have a C++ compiler installed that supports C++ 20 or newer.
 
 If you are building SixtyFPS from source, you need to make sure that your Rust installation is up-to-date. If you have installed Rust using `rustup`, then you can upgrade to the latest Version of Rust by running `rustup update`.
 
@@ -29,3 +29,28 @@ New code:
 sixtyfps::Value args[] = { SharedString("Hello"), 42. };
 instance->invoke_callback("foo", args);
 ```
+
+#### Models
+
+`Model::row_data` returns now a `std::optional<ModelData>` and can thus be used with indices that are out of bounds.
+
+This also means that `Model`s must handle invalid indices and may not crash when a invalid index is passed in.
+
+Old code:
+
+```cpp
+float value = another_model->row_data(2);
+do_something(value)
+```
+
+New code:
+
+```cpp
+// `another_model` is a model that contains floats.
+std::optional<float> value = another_model->row_data(2);
+if (value.has_value()) {
+    do_something(*value);
+} else {
+    // row index 2 is out of bounds
+}
+```