remove null case from explicit binding implementation

Author: jonsequitur

src/System.CommandLine/Binding/MissingValueSource.cs

@@ -0,0 +1,14 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace System.CommandLine.Binding
+{
+    internal class MissingValueSource : IValueSource
+    {
+        public bool TryGetValue(IValueDescriptor valueDescriptor, BindingContext? bindingContext, out object? boundValue)
+        {
+            boundValue = null;
+            return false;
+        }
+    }
+}

src/System.CommandLine/Binding/ModelBinder.cs

@@ -202,12 +202,7 @@ private IReadOnlyList<BoundValue> GetValues(
                 var valueSource = GetValueSource(bindingSources, bindingContext, valueDescriptor);
 
                 BoundValue? boundValue;
-                if (valueSource is null)
-                {
-                    // If there is no source to bind from, no value can be bound.
-                    boundValue = null;
-                }
-                else if (!bindingContext.TryBindToScalarValue(
+                if (!bindingContext.TryBindToScalarValue(
                         valueDescriptor,
                         valueSource,
                         out boundValue) && valueDescriptor.HasDefaultValue)
@@ -240,7 +235,7 @@ private IReadOnlyList<BoundValue> GetValues(
             return values;
         }
 
-        private IValueSource? GetValueSource(
+        private IValueSource GetValueSource(
             IDictionary<IValueDescriptor, IValueSource>? bindingSources,
             BindingContext bindingContext,
             IValueDescriptor valueDescriptor)
@@ -263,7 +258,7 @@ private IReadOnlyList<BoundValue> GetValues(
                 return new ParseResultMatchingValueSource();
             }
 
-            return null;
+            return new MissingValueSource();
         }
 
         public override string ToString() =>