Fix null pointer exception in experimental fluent

[fjtirado]

Following test

 @Test
  void set_with_string() {
    try (WorkflowApplication app = WorkflowApplication.builder().build()) {
      Workflow workflow =
          FuncWorkflowBuilder.workflow()
              .tasks(f -> f.set(s -> s.expr("{message:\"hello world!\"")))
              .build();

      WorkflowDefinition workflowDefinition = app.workflowDefinition(workflow);

      Map<String, Object> output =
          workflowDefinition.instance(Map.of()).start().join().asMap().orElseThrow();

      assertEquals(Map.of("message", "hello world!"), output);
    }

is throwing null pointer exception

This issue is a regression caused by #1170