input: Write a python function to create a pandas DataFrame with specified columns and rows.
  assert create_dataframe([str(i) for i in range(10)], 20).shape == (20, 10)
  assert set(create_dataframe([str(i) for i in range(5)], 15).columns) == set('01234')
  assert create_dataframe(['A', 'B', 'C'], 10).shape == (10, 3)
output: import pandas as pd
  
  def create_dataframe(columns, rows):
      data = {col: [None] * rows for col in columns}
      df = pd.DataFrame(data)
      return df