input: Write a function that takes a SQL query as input and executes it using Python's sqlite3 module. The function should return the results of the query as a list of tuples. If the query string is empty, the function should return None.
  
  def execute_sql_query(query):
      if not query:
          return None
      import sqlite3
      conn = sqlite3.connect('movies.db') 
      cursor = conn.cursor()
      cursor.execute(query)
      result = cursor.fetchall()
      conn.close()
      return result
  
  assert execute_sql_query("SELECT * FROM Movies WHERE category = 'horror'") == [('Movie1', 'horror'), ('Movie2', 'horror')]
  assert execute_sql_query("") == None
  assert execute_sql_query("SELECT * FROM Movies") == [('Movie1', 'horror'), ('Movie2', 'comedy'), ('Movie3', 'drama')]
output: def execute_sql_query(query):
      if not query:
          return None
      import sqlite3
      conn = sqlite3.connect('movies.db') 
      cursor = conn.cursor()
      cursor.execute(query)
      result = cursor.fetchall()
      conn.close()
      return result