Change to metafield type handling in liquid

Effectively immediately, metafields without a type value will return "string" instead of blank. For example, this check will no longer be true

{% if metafield.type == blank %}
  This will no longer render.
{% endif %}

Instead, a value of string will be returned:

{% if metafield.type == 'string' %}
  This metafield is of type `string`.
{% endif %}