Omit falsy attributes from link().

[kmonahan]

Fixes issue where values of FALSE or NULL were passed through as strings instead of removing the attribute.

Example:

{% set link_attributes = {
    'class': link_classes,
    'aria-current': item.is_active ? 'page' : null,
  } %}
{{ link(item.title, item.url, link_attributes) }}

Before, resulted in aria-current="null". After, omits the aria-current attribute when the value is null.