Use undefined instead of null as default value

This fixes the type issues in mapbox#107. Also it makes more sense. I'm unsure about the `return undefined`.

Author: Timmmm

compile.js

@@ -101,7 +101,7 @@ function compileDest(ctx) {
     for (var i = 0; i < ctx._proto.fields.length; i++) {
         var field = ctx._proto.fields[i];
         props[field.name + ': ' + JSON.stringify(ctx._defaults[field.name])] = true;
-        if (field.oneof) props[field.oneof + ': null'] = true;
+        if (field.oneof) props[field.oneof + ': undefined'] = true;
     }
     return '{' + Object.keys(props).join(', ') + '}';
 }
@@ -295,7 +295,7 @@ function getDefaultValue(field, value) {
     case 'string':   return value || '';
     case 'bool':     return value === 'true';
     case 'map':      return {};
-    default:         return null;
+    default:         return undefined;
     }
 }