Null value converted to a number about shapefile

[mbostock]

I intend to investigate.

[mhkeller]

It seems that the property value for avg_temp is ' ' for the null field. Exporting other shapefiles from QGIS looks like it gives null fields a value of '**********'.

When this whitespace string is run through dbf/number.js, +' ' becomes 0 and the conditional is false, returning the value variable as 0.

To catch this scenario you could add the following to number.js but I'm not familiar enough with the dbf spec to know if I have a faulty shapefile or whether this scenario should indeed be handled.

 if (value.trim() === '') {
   return null
 }

[mbostock]

Fixed in #32.