title: Django/Python doesn't like None value for FloatField
body: <p>I have an <code>Account</code> class in Django with an <code>interest_rate</code> attribute (corresponding to <code>account.interest_rate</code> in my database). Here's the declaration for <code>interest_rate</code>:</p>
  
  <pre><code>interest_rate = models.FloatField(null=True, blank=True)
  </code></pre>
  
  <p>If I do something like this, it works:</p>
  
  <pre><code>account = Account()
  account.interest_rate = 5
  account.save()
  </code></pre>
  
  <p>But if I do this, it doesn't work:</p>
  
  <pre><code>account = Account()
  account.interest_rate = None
  account.save()
  </code></pre>
  
  <p>I get this error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "./import.py", line 18, in &lt;module&gt;
      cProfile.run('g.process()', 'prof')
    File "/usr/lib/python2.6/cProfile.py", line 29, in run
      prof = prof.run(statement)
    File "/usr/lib/python2.6/cProfile.py", line 135, in run
      return self.runctx(cmd, dict, dict)
    File "/usr/lib/python2.6/cProfile.py", line 140, in runctx
      exec cmd in globals, locals
    File "&lt;string&gt;", line 1, in &lt;module&gt;
    File "/home/jason/projects/mcifdjango/mcif/models/generic_import.py", line 34, in process
      Account.save_in_bulk(self.rows)
    File "/home/jason/projects/mcifdjango/mcif/models/account.py", line 45, in save_in_bulk
      cursor.execute(Account.bulk_insert_statement(rows))
    File "/usr/lib/pymodules/python2.6/django/db/backends/util.py", line 15, in execute
      return self.cursor.execute(sql, params)
    File "/usr/lib/pymodules/python2.6/django/db/backends/mysql/base.py", line 86, in execute
      return self.cursor.execute(query, args)
    File "/usr/lib/pymodules/python2.6/MySQLdb/cursors.py", line 168, in execute
      if not self._defer_warnings: self._warning_check()
    File "/usr/lib/pymodules/python2.6/MySQLdb/cursors.py", line 82, in _warning_check
      warn(w[-1], self.Warning, 3)
  _mysql_exceptions.Warning: Data truncated for column 'interest_rate' at row 1
  </code></pre>
  
  <p>Why won't it let me save a null on a nullable field? Am I doing it wrong?</p>