MultipleField._is_select returns false if data is None

Currently,
ModelSelectField Multiple 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
Merge pull request #60 from eventsage/master

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