input: Write a function to generate an HTML table displaying information about the top 10 bestselling books in a specific genre from a SQLite database. The function should connect to a database named 'books.db', retrieve the data for the specified genre, and then generate an HTML table to display the information. The table should have columns for Title, Author, Genre, and Sales. If the query returns no results, the function should return an empty string.
  
  assert generate_html_table('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></table>'
  assert generate_html_table('Mystery')=='<table><tr><th>Title</th><th>Author</th><th>Genre</th><th>Sales</th></tr><tr><td>Book3</td><td>Author3</td><td>Mystery</td><td>800</td></tr><tr><td>Book4</td><td>Author4</td><td>Mystery</td><td>700</td></tr></table>'
  assert generate_html_table('Sci-Fi')==''
output: 
  def generate_html_table(genre):
      import sqlite3
      conn = sqlite3.connect('books.db')
      cursor = conn.cursor()
      cursor.execute("SELECT Title, Author, Genre, Sales FROM books WHERE Genre=? ORDER BY Sales DESC LIMIT 10", (genre,))
      rows = cursor.fetchall()
      conn.close()
      
      if not rows:
          return ''
      
      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
  