Here's my current plan:

• If an attribute value is null, include that in the value list and add it to the count.
• If an attribute's only value is null, give it type: 'Null'.
• If an attribute has a null value but also other values, do not let the null value influence the attribute's type: rely on the actual values.

Unfortunately, that first point seems to be blocked by mapnik/node-mapnik#668, which results in nulls being added that weren't part of the original data.