Add cancellation tokens and improve null safety

Enhanced `ProcessAsync` in `ContentQueryExecutor.cs` and `PageContentQueryExecutor.cs` to handle `cancellationToken` for better operation cancellation. Added `System.Diagnostics.CodeAnalysis` namespace and `[return: NotNull]` attribute in `PageContentQueryExecutor.cs` to improve nullability annotations. Modified return statement to return an empty list if `results` is null, preventing potential `NullReferenceException`.

src/XperienceCommunity.DataContext/ContentQueryExecutor.cs

@@ -40,7 +40,7 @@ public async Task<IEnumerable<T>> ExecuteQueryAsync(ContentItemQueryBuilder quer
                 {
                     foreach (var processor in _processors.OrderBy(x=> x.Order))
                     {
-                        await processor.ProcessAsync(result);
+                        await processor.ProcessAsync(result, cancellationToken);
                     }
                 }
 

src/XperienceCommunity.DataContext/PageContentQueryExecutor.cs

@@ -1,4 +1,5 @@
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 using CMS.ContentEngine;
 using CMS.Websites;
 using Microsoft.Extensions.Logging;
@@ -22,6 +23,7 @@ public PageContentQueryExecutor(ILogger<PageContentQueryExecutor<T>> logger,
             _processors = processors?.ToImmutableList();
         }
 
+        [return: NotNull]
         public async Task<IEnumerable<T>> ExecuteQueryAsync(ContentItemQueryBuilder queryBuilder,
             ContentQueryExecutionOptions queryOptions, CancellationToken cancellationToken)
         {
@@ -39,11 +41,11 @@ public async Task<IEnumerable<T>> ExecuteQueryAsync(ContentItemQueryBuilder quer
                 {
                     foreach (var processor in _processors.OrderBy(x => x.Order))
                     {
-                        await processor.ProcessAsync(result);
+                        await processor.ProcessAsync(result, cancellationToken);
                     }
                 }
 
-                return results;
+                return results ?? [];
             }
             catch (Exception ex)
             {