{x:Bind} 函数到 bool? 属性在为空时不更新

Describe the bug

Having an {x:Bind} to a function targeting a bool? property causes the framework not to refresh the binding (ie. to evaluate the function again) when that target property is set to null. Switching back to a converter works as expected instead, but has another whole series of drawbacks, so it's not really a viable solution for this (higher overhead, verbosity, etc.).