input: Write a function to generate an HTML table displaying information about the top 10 bestselling books in a specific genre. The function should connect to a SQLite database, retrieve the data from a table named "Books" with columns id, title, author, genre, sales, and then generate an HTML table based on the retrieved data. If the genre specified is empty or does not exist, the function should return an empty HTML table.
  assert display_top_books('Fantasy')=='<table><tr><th>Title</th><th>Author</th><th>Genre</th><th>Sales</th></tr><tr><td>Book1</td><td>Author1</td><td>Fantasy</td><td>1000</td></tr><tr><td>Book2</td><td>Author2</td><td>Fantasy</td><td>900</td></tr><tr><td>Book3</td><td>Author3</td><td>Fantasy</td><td>800</td></tr><tr><td>...</td><td>...</td><td>Fantasy</td><td>...</td></tr></table>'
  assert display_top_books('Sci-Fi')=='<table><tr><th>Title</th><th>Author</th><th>Genre</th><th>Sales</th></tr><tr><td>BookA</td><td>AuthorA</td><td>Sci-Fi</td><td>950</td></tr><tr><td>BookB</td><td>AuthorB</td><td>Sci-Fi</td><td>850</td></tr><tr><td>...</td><td>...</td><td>Sci-Fi</td><td>...</td></tr></table>'
  assert display_top_books('Mystery')==''
  assert display_top_books('')=='<table></table>'
output: def display_top_books(genre):
      import sqlite3
      conn = sqlite3.connect('books.db')
      cursor = conn.cursor()
      if genre:
          cursor.execute("SELECT title, author, genre, sales FROM Books WHERE genre=? ORDER BY sales DESC LIMIT 10", (genre,))
      else:
          return '<table></table>'
      rows = cursor.fetchall()
      conn.close()
      if not rows:
          return '<table></table>'
      html = '<table><tr><th>Title</th><th>Author</th><th>Genre</th><th>Sales</th></tr>'
      for row in rows:
          html += f'<tr><td>{row[0]}</td><td>{row[1]}</td><td>{row[2]}</td><td>{row[3]}</td></tr>'
      html += '</table>'
      return html