MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:vm="clr-namespace:BindingProblem"
             x:Class="BindingProblem.MainPage"
             x:DataType="{x:Type vm:ViewModel}">

    <ScrollView>
        <VerticalStackLayout
            Spacing="25"
            Padding="30,0"
            VerticalOptions="Center">

            <Label
                Text="{Binding Sources}"
                HorizontalOptions="Center">
                <Label.Triggers>
                    <DataTrigger TargetType="Label" Binding="{Binding Sources}" Value="">
                        <Setter Property="Text" Value="Field is Empty"/>
                    </DataTrigger>
                </Label.Triggers>
            </Label>

            <Button
                Text="Click me"
                Command="{Binding NextPageCommand}}"
                HorizontalOptions="Center" />

        </VerticalStackLayout>
    </ScrollView>

</ContentPage>