Setting an attribute to null (and only null) removes that attribute

Small change to ensure that null attribute values _remove_ those attributes.

Example template: `<input disabled="{{isDisabled}}">`. When `isDisabled` is null (and only null), the output is `<input>`. Any other value will insert that as the attribute's value in the output.

This addresses #68

This also paves the way to add a `{{boolean-attribute}}` helper