Module does not properly handle 0 ( zero ) as default value.

Have a use case where I'd like an integer dropdown to display integers from 0 to 20 with a default of 0. Module is unable to save zero as a default, errors when no value is selected ( i.e. if `Select a value` is left in the dropdown).

Default value issue problem most likely in _select_number_widget_field_widget_form:

  if (!empty($default_value)) {
    $widget_element['#default_value'] = $default_value;
  }

Changing `empty()` to `isset()` will cause default to be assigned, but was still unable to get a 0 to successfully save when selected. Ultimately reverted back to an input field.

Steps to recreate:
1. Create an integer field
2. Set min to 0, max to 20, step 1, default 0
3. Save the field, then edit. Zero not selected as default.
4. Create a node, then select 0 as value, or leave with `Select a value` selected.