Fix rendering issue when rendered variable is a boolean set to false

The rendering function does not work when the variable to render is a boolean at false. I guess the aim was to make sure the property exists, hence the additional check for 0's. My proposal to use `typeof` makes this 2nd check useless.

The use-case is to control css classes, e.g.:

    <span class="wtb-opt-yesno wtb-opt-selected-{drive}">