Prevent type casting nil values in Go collections (microsoft#2766)

* Add nil check before typecasting in go collections

* Update changelog

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Deprecated Visual Studio OpenAPI reference packages.
 - Fixes a bug where a stackoverflow exception occurs when inlined schemas have self-references [2656](https://github.com/microsoft/kiota/issues/2656)
+- Fixes nil safety while type casting values in collections in Go
 
 ## [1.3.0] - 2023-06-09
 

src/Kiota.Builder/Writers/Go/CodeMethodWriter.cs

@@ -663,10 +663,12 @@ private static void WriteCollectionCast(string propertyTypeImportName, string so
         writer.WriteLines($"{targetVarName} := make([]{propertyTypeImportName}, len({sourceVarName}))",
             $"for i, v := range {sourceVarName} {{");
         writer.IncreaseIndent();
+        writer.StartBlock("if v != nil {");
         var derefPrefix = dereference ? "*(" : string.Empty;
         var derefSuffix = dereference ? ")" : string.Empty;
         writer.WriteLine($"{targetVarName}[i] = {GetTypeAssertion(derefPrefix + "v", pointerSymbol + propertyTypeImportName)}{derefSuffix}");
         writer.CloseBlock();
+        writer.CloseBlock();
     }
 
     private static string getSendMethodName(string returnType, CodeMethod codeElement, bool isPrimitive, bool isBinary, bool isEnum)
@@ -919,12 +921,14 @@ private void WriteSerializationMethodCall(CodeTypeBase propType, CodeElement par
             writer.WriteLines($"cast := make([]{parsableSymbol}, len({valueGet}))",
                 $"for i, v := range {valueGet} {{");
             writer.IncreaseIndent();
+            writer.StartBlock("if v != nil {");
             if (isInterface)
                 writer.WriteLine($"cast[i] = {GetTypeAssertion("v", parsableSymbol)}");
             else
                 writer.WriteLines("temp := v", // temporary creating a new reference to avoid pointers to the same object
                     $"cast[i] = {parsableSymbol}(&temp)");
             writer.CloseBlock();
+            writer.CloseBlock();
         }
         var collectionPrefix = propType.IsCollection ? "CollectionOf" : string.Empty;
         var collectionSuffix = propType.IsCollection ? "s" : string.Empty;

tests/Kiota.Builder.Tests/Writers/Go/CodeMethodWriterTests.cs

@@ -799,6 +799,7 @@ public void WritesModelFactoryBodyForUnionModels()
         Assert.Contains("result.SetStringValue(val)", result);
         Assert.Contains("else if val, err := parseNode.GetCollectionOfObjectValues(CreateComplexType2FromDiscriminatorValue); val != nil {", result);
         Assert.Contains("cast := make([]ComplexType2, len(val))", result);
+        Assert.Contains("if v != nil ", result);
         Assert.Contains("for i, v := range val", result);
         Assert.Contains("result.SetComplexType2Value(cast)", result);
         Assert.Contains("return result, nil", result);