change empty next_page_token value to None

Author: lkawka

src/a2a/server/tasks/database_task_store.py

@@ -187,7 +187,7 @@ async def list(
             next_page_token = (
                 str(page_number + 1)
                 if total_count > (page_number + 1) * page_size
-                else ''
+                else None
             )
 
             return TasksPage(

src/a2a/server/tasks/inmemory_task_store.py

@@ -72,7 +72,7 @@ async def list(
         next_page_token = (
             str(page_token + 1)
             if (page_token + 1) * page_size < total_size
-            else ''
+            else None
         )
 
         return TasksPage(

tests/server/tasks/test_database_task_store.py

@@ -181,7 +181,7 @@ async def test_get_task(db_store_parameterized: DatabaseTaskStore) -> None:
             ListTasksParams(),
             ['task-4', 'task-3', 'task-2', 'task-1', 'task-0'],
             5,
-            '',
+            None,
         ),
         # Pagination (first page)
         (
@@ -198,20 +198,25 @@ async def test_get_task(db_store_parameterized: DatabaseTaskStore) -> None:
             '2',
         ),
         # Filtering by context_id
-        (ListTasksParams(context_id='context-1'), ['task-3', 'task-1'], 2, ''),
+        (
+            ListTasksParams(context_id='context-1'),
+            ['task-3', 'task-1'],
+            2,
+            None,
+        ),
         # Filtering by status
         (
             ListTasksParams(status=TaskState.working),
             ['task-3', 'task-1'],
             2,
-            '',
+            None,
         ),
         # Combined filtering (context_id and status)
         (
             ListTasksParams(context_id='context-0', status=TaskState.submitted),
             ['task-2', 'task-0'],
             2,
-            '',
+            None,
         ),
         # Combined filtering and pagination
         (

tests/server/tasks/test_inmemory_task_store.py

@@ -41,7 +41,7 @@ async def test_in_memory_task_store_get_nonexistent() -> None:
             ListTasksParams(),
             ['task-0', 'task-1', 'task-2', 'task-3', 'task-4'],
             5,
-            '',
+            None,
         ),
         # Pagination (first page)
         (
@@ -58,20 +58,25 @@ async def test_in_memory_task_store_get_nonexistent() -> None:
             '2',
         ),
         # Filtering by context_id
-        (ListTasksParams(context_id='context-1'), ['task-1', 'task-3'], 2, ''),
+        (
+            ListTasksParams(context_id='context-1'),
+            ['task-1', 'task-3'],
+            2,
+            None,
+        ),
         # Filtering by status
         (
             ListTasksParams(status=TaskState.working),
             ['task-1', 'task-3'],
             2,
-            '',
+            None,
         ),
         # Combined filtering (context_id and status)
         (
             ListTasksParams(context_id='context-0', status=TaskState.submitted),
             ['task-0', 'task-2'],
             2,
-            '',
+            None,
         ),
         # Combined filtering and pagination
         (