fix(task): handle null configuration in TaskEngine.execute

Add null check for entity configuration and tasks list to prevent
NullPointerException when TaskEngine.execute is called before
entity configuration is set (e.g., during beforeTokenValidation phases).

Author: christophechevalier

src/main/java/io/github/linagora/linid/im/plugin/task/TaskEngineImpl.java

@@ -65,7 +65,11 @@ public class TaskEngineImpl implements TaskEngine {
 
   @Override
   public void execute(DynamicEntity dynamicEntity, TaskExecutionContext context, String phase) {
-    dynamicEntity.getConfiguration().getTasks()
+    var configuration = dynamicEntity.getConfiguration();
+    if (configuration == null) {
+      return;
+    }
+    configuration.getTasks()
         .stream()
         .filter(task -> task.getPhases().contains(phase))
         .map(this::mergeConfigurationWithGlobal)

src/test/java/io/github/linagora/linid/im/plugin/task/TaskEngineImplTest.java

@@ -210,6 +210,20 @@ void testExecuteWithNullTasks() {
     Mockito.verifyNoInteractions(configurationService);
   }
 
+  @Test
+  @DisplayName("test execute: should not throw when configuration is null")
+  void testExecuteWithNullConfiguration() {
+    var entity = Mockito.mock(DynamicEntity.class);
+    Mockito.when(entity.getConfiguration()).thenReturn(null);
+
+    // Should not throw NullPointerException
+    taskEngine.execute(entity, new TaskExecutionContext(), "myphase");
+
+    // Verify no plugin interaction occurred
+    Mockito.verifyNoInteractions(taskRegistry);
+    Mockito.verifyNoInteractions(configurationService);
+  }
+
   public static class DummyPlugin implements TaskPlugin {
     @Override
     public boolean supports(@NonNull String type) {