input: Write a function to create a Django application with a model that stores the name, address, and email of a user. The function should return the User model class. If the given project name is empty, the function should return None.
  from django.db import models
  
  def create_user_model(project_name):
      if not project_name:
          return None
      class User(models.Model): 
          name = models.CharField(max_length=100) 
          address = models.CharField(max_length=100)
          email = models.EmailField(max_length=100)
      return User
  
  assert create_user_model("myapp") is not None
  assert create_user_model("") is None
output: from django.db import models
  
  def create_user_model(project_name):
      if not project_name:
          return None
      class User(models.Model): 
          name = models.CharField(max_length=100) 
          address = models.CharField(max_length=100)
          email = models.EmailField(max_length=100)
      return User