Apex JSON.serialize() with null values (RELOADED)

Nulls are being rendered in the latest JSON (32.0/33.0). You must explicitly the set the value to null for it to work. Consider the following code:

Account a = new Account(Name='test',Industry=null);
System.debug(LoggingLevel.ERROR, JSON.serialize(a));

Output:

{"attributes":{"type":"Account"},"Name":"test","Industry":null}

If you need every single field, you'll have to do something like:

Account a = new Account();
for(SObjectField f: account.sobjecttype.getdescribe().fields.getmap().values()) {
    try { a.put(f, null); } catch(exception e) { }
}

Even better, you could always just use a Map, which avoids the errors of field editability:

Map<String, Object> a = new Map<String, Object>();
SObject b = [SELECT Id, Name FROM Account LIMIT 1];
for(SObjectField f: account.sobjecttype.getdescribe().fields.getmap().values()) {
    try { a.put(String.valueOf(f), b.get(f)); } 
    catch(Exception e) { a.put(String.valueOf(f), null); }
}
System.debug(logginglevel.error, json.serialize(a));

I've just tried this code in executeAnonymous and came up with fully realized JSON. Note that I use a try-catch here because queried SObjects generate errors for missing fields. If you're using a manually constructed SObject, you'll not have to worry about catching errors.

Finally, note that executeAnonymous tends to always run in the latest version. You won't normally see differences in execution contexts because the entire transaction is set to version 32.0 mode when running executeAnonymous. To emulate a lower version, you'd have to write a Visualforce page and set the version of the page to a lower value, and then also set the controller's version to that lower value, etc...

Alternatively, you could write a REST class, and call that REST function. That should also set the version correctly. Since executeAnonymouse is basically an eval(), it tends to behave in the latest version, because it has to be compiled on demand.