Issue #59: MultipleField._is_select returns false if data is None 

Currently,
ModelSelectFieldMultiple returns Error if field exists but has None value

This occur when creating a new item,
as the object has not been created on page load.

Modified to,
QuerySetSelectMultipleField._is_select returns False
when the items data selected from is None.

Added Test,
test_forms
  .WTFormsAppTestCase
    .test_modelselectfield_multiple_initalvalue_None

closes issue #59