Fix the C++ build

Remove the Optional from the evaluation context passing for property
evaluation. Unfortunately there are nullptr uses left on the C++ side,
that need to be replaced with passing through.

examples/cpptest/main.cpp

@@ -10,12 +10,12 @@ int main() {
 
     component.plus_clicked.set_handler([](auto...){
         auto &counter = component.counter;
-        counter.set(counter.get() + 1);
+        counter.set(counter.get(nullptr) + 1);
     });
 
     component.minus_clicked.set_handler([](auto...){
         auto &counter = component.counter;
-        counter.set(counter.get() - 1);
+        counter.set(counter.get(nullptr) - 1);
     });
 
     sixtyfps::run(&component);