Fallback if column has only null

[ximex]

If a column has only null values we cant calc the size for the buffer.
So i think a fallback to String would be fine.
Example:

function obj(_) {
    var fields = {}, o = [];
    for (var p in _) fields[p] = typeof _[p];
    for (var n in fields) {
        var t = types[fields[n]];
        o.push({
            name: n,
            type: t || 'C',
            size: fieldSize[t] || fieldSize['C']
        });
    }
    return o;
}