Add StringNullOrEmptyBoolConverter to check when string value is null or empty

src/Mobile/eShopOnContainers/eShopOnContainers.Core/App.xaml

@@ -110,6 +110,7 @@
             <converters:ToUpperConverter x:Key="ToUpperConverter" />
             <converters:WebNavigatingEventArgsConverter x:Key="WebNavigatingEventArgsConverter" />
             <converters:WebNavigatedEventArgsConverter x:Key="WebNavigatedEventArgsConverter" />
+            <converters:StringNullOrEmptyBoolConverter x:Key="StringNullOrEmptyBoolConverter" />
 
             <!-- STYLES -->
             <Style x:Key="ValidationErrorLabelStyle"

src/Mobile/eShopOnContainers/eShopOnContainers.Core/Converters/StringNullOrEmptyBoolConverter.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Globalization;
+using Xamarin.Forms;
+
+namespace eShopOnContainers.Core.Converters
+{
+    public class StringNullOrEmptyBoolConverter : IValueConverter
+    {
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            var stringValue = value as string;
+
+            return !string.IsNullOrEmpty(stringValue);
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            return null;
+        }
+    }
+}