Don't display [None] twice.

The empty list item was getting added twice: once by the XAML and once
in PullRequestListViewModel. This removes the XAML empty list item code
as I assume it's supposed to be set in the VM. Fixes #390.

Author: grokys

src/GitHub.VisualStudio/UI/Views/PullRequestListView.xaml

@@ -214,17 +214,9 @@
 
               <ListBox x:Name="assignees"
                        DockPanel.Dock="Top"
+                       ItemsSource="{Binding Assignees}"
                        SelectedItem="{Binding SelectedAssignee}"
                        Style="{DynamicResource GitHubPopupThing}">
-                <ListBox.ItemsSource>
-                  <MultiBinding Converter="{ui:StickieListItemConverter}">
-                    <Binding Path="EmptyUser" />
-                    <Binding Path="Assignees" />
-                  </MultiBinding>
-                </ListBox.ItemsSource>
-                <i:Interaction.Behaviors>
-                  <ui:AddEmptyItemToList />
-                </i:Interaction.Behaviors>
                 <i:Interaction.Triggers>
                   <i:EventTrigger EventName="SelectionChanged">
                     <ui:ClosePopupAction TargetName="assigneePopup" />
@@ -263,17 +255,9 @@
 
               <ListBox x:Name="authors"
                        DockPanel.Dock="Top"
+                       ItemsSource="{Binding Authors}"
                        SelectedItem="{Binding SelectedAuthor}"
                        Style="{DynamicResource GitHubPopupThing}">
-                <ListBox.ItemsSource>
-                  <MultiBinding Converter="{ui:StickieListItemConverter}">
-                    <Binding Path="EmptyUser" />
-                    <Binding Path="Authors" />
-                  </MultiBinding>
-                </ListBox.ItemsSource>
-                <i:Interaction.Behaviors>
-                  <ui:AddEmptyItemToList />
-                </i:Interaction.Behaviors>
                 <i:Interaction.Triggers>
                   <i:EventTrigger EventName="SelectionChanged">
                     <ui:ClosePopupAction TargetName="authorPopup" />