Annotate empty string locale code enum with cause for reporting

Without context, the meaning of "empty string" reports is difficult to interpret when looking at UMA data. Add an annotation to this bucket to explain how it is gets reported as a page source language.

Bug: 1426907
Change-Id: I1af3d886bea66705a8644ed6e82bcc89e4c43ce2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4362220
Reviewed-by: Trevor Perrier <perrier@chromium.org>
Auto-Submit: Anthony Cui <cuianthony@chromium.org>
Commit-Queue: Anthony Cui <cuianthony@chromium.org>
Commit-Queue: Trevor Perrier <perrier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1120759}