code: def test_prompt_display(self):
      skills_manager = SkillsManager()
      skill1 = Mock()
      skill2 = Mock()
      skill1.name = "SkillA"
      skill2.name = "SkillB"
      skill1.print = "SkillA"
      skill2.print = "SkillB"
      skills_manager.add_skills(skill1, skill2)
  
      # Test prompt_display method when skills exist
      prompt = skills_manager.prompt_display()
      assert (
          "You can call the following functions that have been pre-defined for you:"
          in prompt
      )
  
      # Test prompt_display method when no skills exist
      skills_manager._skills = []
      prompt = skills_manager.prompt_display()
      assert prompt is None
  
imports: from typing import Optional
  from unittest.mock import MagicMock, Mock, patch
  import uuid
  import pandas
  import pytest
  from pandasai.agent import Agent
  from pandasai.helpers.code_manager import CodeExecutionContext, CodeManager
  from pandasai.helpers.skills_manager import SkillsManager
  from pandasai.llm.fake import FakeLLM
  from pandasai.skills import skill, Skill
  from pandasai.smart_dataframe import SmartDataframe
label_desc: Use image node_id 11 for calling the TestSkills obj's underlying member method code with example usage: obj.test_prompt_display() without return types