omit attributes that are logically false

HTML attributes that have a value are considered logically true, even if the value is the empty string.  The prior code translates an element like `[:option {:selected false} ...]` to `<option selected="false">`, which the browser interprets as true.

Omitting the attribute altogether if the value is logically false (i.e. false or nil) fixes this.

Note: this pull request only touches `template/add-attr!`, `template-compile` may require a similar change.