Fix null annotations on collection binding

Author: daningalla

src/generator/Vertical/Cli/SourceGenerator/Utilities/BindingExpressionHelper.cs

@@ -119,10 +119,22 @@ private string GetCollectionValueExpression(
     {
         var elementConversionExpression = GetConversionExpression(valueType);
         var formattedExpression = string.Format(collectionConversionTemplate, valueType);
+        var isNullableCollection = collectionType is
+        {
+            IsReferenceType: true,
+            NullableAnnotation: NullableAnnotation.Annotated
+        };
+        var contextMethod = isNullableCollection
+            ? "GetCollectionValueOrDefault"
+            : "GetCollectionValue";
+        var resolvedCollectionType = isNullableCollection
+            ? collectionType.WithNullableAnnotation(NullableAnnotation.NotAnnotated)
+            : collectionType;
+        var bindingNameAnnotation = isNullableCollection ? "!" : null;
 
         return elementConversionExpression != "null"
-            ? $"{contextParameter}.GetCollectionValue(x => x.{bindingName}, {elementConversionExpression}, {formattedExpression})"
-            : $"{contextParameter}.GetCollectionValue<{valueType}, {collectionType}>(x => x.{bindingName}, null, {formattedExpression})";
+            ? $"{contextParameter}.{contextMethod}<{valueType}, {resolvedCollectionType}>(x => x.{bindingName}{bindingNameAnnotation}, {elementConversionExpression}, {formattedExpression})"
+            : $"{contextParameter}.{contextMethod}<{valueType}, {resolvedCollectionType}>(x => x.{bindingName}{bindingNameAnnotation}, null, {formattedExpression})";
     }
 
     private string GetConversionExpression(ITypeSymbol valueType)

src/lib/Vertical/Cli/Binding/BindingContext.cs

@@ -106,7 +106,40 @@ public TCollection GetCollectionValue<TElement, TCollection>(
         if (TryGetCollectionParseResultValue(binding, elementConverter, createCollection, out var collection))
             return collection;
 
-        return bindingArgs.TryGetDefaultValue(out collection) ? collection : default!;
+        return bindingArgs.TryGetDefaultValue(out collection) 
+            ? collection 
+            : createCollection([]);
+    }
+    
+    /// <summary>
+    /// Gets a collection property value.
+    /// </summary>
+    /// <param name="propertyExpression">The expression that identifies the model property being bound.</param>
+    /// <param name="elementConverter">A function that converts string arguments to the target element type.</param>
+    /// <param name="createCollection">A function that converts enumerable values to the strong collection type.</param>
+    /// <typeparam name="TElement">The collection or array's element type.</typeparam>
+    /// <typeparam name="TCollection">The collection type.</typeparam>
+    /// <returns>The collection value</returns>
+    public TCollection? GetCollectionValueOrDefault<TElement, TCollection>(
+        Expression<Func<TModel, TCollection>> propertyExpression,
+        ValueConverter<TElement>? elementConverter,
+        Func<IEnumerable<TElement>, TCollection> createCollection)
+        where TCollection : IEnumerable<TElement>
+    {
+        var binding = GetPropertyBinding(propertyExpression);
+        var bindingArgs = new PropertyBinder<TCollection>(binding, this, null);
+
+        binding.TryBindValue(bindingArgs);
+
+        if (bindingArgs.TryGetExplicitValue(out var value))
+            return value;
+
+        if (TryGetCollectionParseResultValue(binding, elementConverter, createCollection, out var collection))
+            return collection;
+
+        return bindingArgs.TryGetDefaultValue(out collection)
+            ? collection
+            : default;
     }
 
     /// <summary>