title: Checking for nulls with sqlalchemy
body: <p>I can never seem to get the filter calls right when I want to check for null values.</p>
  
  <p>Is this function unnecessary?    </p>
  
  <pre><code>def sqla_false(field):
      if field.type.__class__ == db.String:
          return db.or_(field == None, field == '')
      return field == None
  </code></pre>
  
  <p>How does sqlalchemy store unset fields? 
  What if one of my wtforms forms sends a "" as a value for one of the fields, would NULL or "" be stored in the database when I call form.populate_obj(obj)?</p>