Don't convert null to the string "null" (#39)

I hope this is an acceptable change, although I don't know the complete requirements of this package, but this fix will solve mapbox/mapbox-gl-js#8497.

I also have a hard time thinking of a reason it would make sense to have null as a string `"null"` because of the ambiguity it brings.

Co-authored-by: Vladimir Agafonkin <agafonkin@gmail.com>

index.js

@@ -104,7 +104,7 @@ function writeProperties (context, pbf) {
 
     var value = feature.properties[key]
     var type = typeof value
-    if (type !== 'string' && type !== 'boolean' && type !== 'number') {
+    if (value !== null && type !== 'string' && type !== 'boolean' && type !== 'number') {
       value = JSON.stringify(value)
     }
     var valueKey = type + ':' + value