fix ReplaceRenderer class. Method Parse, NullReferenceException

lukencode#356

Author: JohnCampionJr

src/FluentEmail.Core/Defaults/ReplaceRenderer.cs

@@ -10,7 +10,7 @@ public string Parse<T>(string template, T model, bool isHtml = true)
         {
             foreach (PropertyInfo pi in model.GetType().GetRuntimeProperties())
             {
-                template = template.Replace($"##{pi.Name}##", pi.GetValue(model, null).ToString());
+                template = template.Replace($"##{pi.Name}##", pi.GetValue(model, null)?.ToString());
             }
 
             return template;            

test/FluentEmail.Core.Tests/ReplaceRendererTest.cs

@@ -0,0 +1,25 @@
+using FluentEmail.Core.Defaults;
+using FluentEmail.Core.Interfaces;
+using FluentEmail.Core.Models;
+using NUnit.Framework;
+
+namespace FluentEmail.Core.Tests
+{
+    [TestFixture]
+    public class ReplaceRendererTest
+    {
+        [Test]
+        public void ModelPropertyValueIsNull_Test()
+        {
+            ITemplateRenderer templateRenderer = new ReplaceRenderer();
+
+            var address = new Address("james@test.com", "james");
+            Assert.True(address.Name == "james");
+            var template = "this is name: ##Name##";
+            Assert.True("this is name: james" == templateRenderer.Parse(template, address));
+
+            address.Name = null;
+            Assert.True("this is name: " == templateRenderer.Parse(template, address));
+        }
+    }
+}