fix: adds missing null propagation operators for callback and header references

Signed-off-by: Vincent Biret <[email protected]>

Author: baywet

src/Microsoft.OpenApi/Models/References/OpenApiCallbackReference.cs

@@ -85,10 +85,10 @@ internal OpenApiCallbackReference(OpenApiCallback target, string referenceId)
         }
 
         /// <inheritdoc/>
-        public Dictionary<RuntimeExpression, OpenApiPathItem> PathItems { get => Target.PathItems; }
+        public Dictionary<RuntimeExpression, OpenApiPathItem> PathItems { get => Target?.PathItems; }
 
         /// <inheritdoc/>
-        public IDictionary<string, IOpenApiExtension> Extensions { get => Target.Extensions; }
+        public IDictionary<string, IOpenApiExtension> Extensions { get => Target?.Extensions; }
 
         /// <inheritdoc/>
         public void SerializeAsV3(IOpenApiWriter writer)

src/Microsoft.OpenApi/Models/References/OpenApiHeaderReference.cs

@@ -97,37 +97,37 @@ public string Description
         }
 
         /// <inheritdoc/>
-        public bool Required { get => Target.Required; }
+        public bool Required { get => Target?.Required ?? default; }
 
         /// <inheritdoc/>
-        public bool Deprecated { get => Target.Deprecated; }
+        public bool Deprecated { get => Target?.Deprecated ?? default; }
 
         /// <inheritdoc/>
-        public bool AllowEmptyValue { get => Target.AllowEmptyValue; }
+        public bool AllowEmptyValue { get => Target?.AllowEmptyValue ?? default; }
 
         /// <inheritdoc/>
-        public OpenApiSchema Schema { get => Target.Schema; }
+        public OpenApiSchema Schema { get => Target?.Schema; }
 
         /// <inheritdoc/>
-        public ParameterStyle? Style { get => Target.Style; }
+        public ParameterStyle? Style { get => Target?.Style; }
 
         /// <inheritdoc/>
-        public bool Explode { get => Target.Explode; }
+        public bool Explode { get => Target?.Explode ?? default; }
 
         /// <inheritdoc/>
-        public bool AllowReserved { get => Target.AllowReserved; }
+        public bool AllowReserved { get => Target?.AllowReserved ?? default; }
 
         /// <inheritdoc/>
-        public JsonNode Example { get => Target.Example; }
+        public JsonNode Example { get => Target?.Example; }
 
         /// <inheritdoc/>
-        public IDictionary<string, IOpenApiExample> Examples { get => Target.Examples; }
+        public IDictionary<string, IOpenApiExample> Examples { get => Target?.Examples; }
 
         /// <inheritdoc/>
-        public IDictionary<string, OpenApiMediaType> Content { get => Target.Content; }
+        public IDictionary<string, OpenApiMediaType> Content { get => Target?.Content; }
 
         /// <inheritdoc/>
-        public IDictionary<string, IOpenApiExtension> Extensions { get => Target.Extensions; }
+        public IDictionary<string, IOpenApiExtension> Extensions { get => Target?.Extensions; }
 
         /// <inheritdoc/>
         public void SerializeAsV31(IOpenApiWriter writer)