Fix NRE when setting null value in context

Fluid.Tests/TemplateContextTests.cs

@@ -126,6 +126,14 @@ public void CaptureShouldUpdateContext()
             Assert.Contains("greetings", context.ValueNames);
         }
 
+        [Fact]
+        public void ScopeSetValueAcceptsNull()
+        {
+            var context = new TemplateContext();
+            context.SetValue("text", null);
+            Assert.Equal(NilValue.Instance, context.GetValue("text"));
+        }
+
         private class TestClass
         {
             public string Name { get; set; }

Fluid/Scope.cs

@@ -58,7 +58,7 @@ public void SetValue(string name, FluidValue value)
         {
             _properties ??= new Dictionary<string, FluidValue>();
 
-            _properties[name] = value;
+            _properties[name] = value ?? NilValue.Instance;
         }
 
         public Scope EnterChildScope(Scope parent = null)

Fluid/TemplateContext.cs

@@ -237,6 +237,11 @@ public static TemplateContext SetValue(this TemplateContext context, string name
 
         public static TemplateContext SetValue(this TemplateContext context, string name, object value)
         {
+            if (value == null)
+            {
+                return context.SetValue(name, NilValue.Instance);
+            }
+
             return context.SetValue(name, FluidValue.Create(value, context.Options));
         }