Support selecting options with falsy values

Previously you could not select an option which had the number 0
as its value, because there was a check which would detect it as
an option without a value. There was a similar issue with selecting
options which had a blank string as the value.

This closes select2#4604.
This closes select2#4516.
This closes select2#3252.
This closes select2#3519.

This closes select2#4605.
This closes select2#4517.

Author: Mike Dearman

src/js/select2/data/select.js

@@ -170,7 +170,7 @@ define([
       }
     }
 
-    if (data.id) {
+    if (data.id !== undefined) {
       option.value = data.id;
     }