Update parse-row.js

This change fixes this issue – yuanqing#6

src/parse-row.js

@@ -5,7 +5,7 @@ function identity (object) {
 module.exports = function (schema, row, transform) {
   transform = transform || identity
   return row.reduce(function (result, cell, index) {
-    result[schema[index]] = transform(typeof cell === 'object' ? cell.v : null)
+    result[schema[index]] = transform(!!cell ? cell.v : null)
     return result
   }, {})
 }