[new compiler] Cleanup empty string handling

[new compiler] Cleanup empty string handling

Ensure that the empty string always has index 0, that simplifies the code in a
few places and makes it easier to check for the empty string in other places
where there's no access to the string pool itself.

Change-Id: Icd204aec478e8350ef3fee75d89bda1f88cffe26
Reviewed-by: Lars Knoll <lars.knoll@digia.com>