fix: adds missing null prop operator to proxy properties

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

Author: baywet

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

@@ -59,16 +59,16 @@ public string Description
         }
 
         /// <inheritdoc/>
-        public IDictionary<OperationType, OpenApiOperation> Operations { get => Target.Operations; }
+        public IDictionary<OperationType, OpenApiOperation> Operations { get => Target?.Operations; }
 
         /// <inheritdoc/>
-        public IList<OpenApiServer> Servers { get => Target.Servers; }
+        public IList<OpenApiServer> Servers { get => Target?.Servers; }
 
         /// <inheritdoc/>
-        public IList<IOpenApiParameter> Parameters { get => Target.Parameters; }
+        public IList<IOpenApiParameter> Parameters { get => Target?.Parameters; }
 
         /// <inheritdoc/>
-        public IDictionary<string, IOpenApiExtension> Extensions { get => Target.Extensions; }
+        public IDictionary<string, IOpenApiExtension> Extensions { get => Target?.Extensions; }
 
         /// <inheritdoc/>
         public override IOpenApiPathItem CopyReferenceAsTargetElementWithOverrides(IOpenApiPathItem source)