NumberWidget.render() returns None as string

[matthewhegarty]

Problem

NumberWidget.render() returns 'None' as string if value is None and coerce to string is set.

Solution

Add specific check for this

Acceptance Criteria

Test added.

[coveralls]

coverage: 100.0%. remained the same when pulling 315c97a on matthewhegarty:number-widget-coerce-to-string into 77d659d on django-import-export:main.

[pokken-magic]

Nice. All this consistency in handling nulls should be really useful

[matthewhegarty]

Thanks for reviewing. There is some way to go to introduce consistency because of the quirks that have evolved over the last decade, but we are moving in the right direction.