Detect undefined variables

Fluid/Ast/MemberExpression.cs

@@ -44,7 +44,13 @@ public override ValueTask<FluidValue> EvaluateAsync(TemplateContext context)
             {
                 if (context.Model == null)
                 {
-                    return new ValueTask<FluidValue>(value);
+                    // Check equality as IsNil() is also true for EmptyValue
+                    if (context.Undefined is not null && value == NilValue.Instance)
+                    {
+                        return context.Undefined.Invoke(initial.Identifier);
+                    }
+
+                    return new ValueTask<FluidValue>(NilValue.Instance);
                 }
 
                 start = 0;

Fluid/Parser/FluidTemplate.cs

@@ -1,5 +1,5 @@
-using Fluid.Ast;
-using System.Text.Encodings.Web;
+using System.Text.Encodings.Web;
+using Fluid.Ast;
 
 namespace Fluid.Parser
 {

Fluid/Scope.cs

@@ -1,5 +1,5 @@
-using Fluid.Values;
 using System.Runtime.CompilerServices;
+using Fluid.Values;
 
 namespace Fluid
 {