input: Write a function to generate sample data for a web application using Python's faker library. The output should contain three columns of data: name, email, and age. If the input is 0 or less, return None. Otherwise, generate the specified number of rows with sample data.
  from faker import Faker
  def generate_sample_data(num_rows):
      if num_rows <= 0:
          return None
      fake = Faker()
      data = []
      for _ in range(num_rows):
          data.append((fake.name(), fake.email(), fake.random_int(min=18, max=60)))
      return data
  assert generate_sample_data(10) is not None
  assert generate_sample_data(0) is None
  assert generate_sample_data(-5) is None
  assert len(generate_sample_data(15)) == 15
output: 
  def generate_sample_data(num_rows):
      if num_rows <= 0:
          return None
      fake = Faker()
      data = []
      for _ in range(num_rows):
          data.append((fake.name(), fake.email(), fake.random_int(min=18, max=60)))
      return data
  