Allow Binding<Case?> to write into nil (pointfreeco#54)

Fixes pointfreeco#53.

Author: stephencelis

Sources/SwiftUINavigation/Binding.swift

@@ -59,7 +59,6 @@ extension Binding {
     .init(
       get: { self.wrappedValue.flatMap(casePath.extract(from:)) },
       set: { newValue, transaction in
-        guard self.wrappedValue != nil else { return }
         self.transaction(transaction).wrappedValue = newValue.map(casePath.embed)
       }
     )