Heuristic tests: Gracefully handle select options without value or label

The code currently assumes that every select <option> in the input JSON
has a value and label. Otherwise, the test crashes. While testing Greek
forms, this was not the case. Let's default to an empty string if a
value is not available.

Bug: 395061395
Change-Id: I06bd5de272032534b7b3959b3190ca3a0980aba5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6722515
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1484945}