Hide Add Task button if current relation is empty to fix anitab-org…

…#128

I have used `@ViewBuilder` attribute to pass the appropriate `View` to the `navigationBarItems(trailing:)` method. Before collecting data from the server the `id` of `currentRelation` is 0 and if the user doesn't have any Relation then the server response for `id` would be nil. This is why I have checked both of the conditions.

mentorship ios/Views/Relation/Relation.swift

@@ -65,6 +65,14 @@ struct Relation: View {
             }
         }
     }
+
+    @ViewBuilder func trailingNavigationBarItem() -> some View {
+        if let id = relationViewModel.currentRelation.id, id != 0 {
+            Button(LocalizableStringConstants.addTask) {
+                self.relationViewModel.addTask.toggle()
+            }
+        } 
+    }
 
     var body: some View {
         NavigationView {
@@ -116,9 +124,7 @@ struct Relation: View {
             }
             .environment(\.horizontalSizeClass, .regular)
             .navigationBarTitle("Current Relation")
-            .navigationBarItems(trailing: Button(LocalizableStringConstants.addTask) {
-                self.relationViewModel.addTask.toggle()
-            })
+            .navigationBarItems(trailing: trailingNavigationBarItem())
             .sheet(isPresented: $relationViewModel.addTask) {
                 AddTask(relationViewModel: self.relationViewModel)
             }