[json-schema-compiler] Support abs::optional<double>

Following up the addition of optional support for other values, this CL
adds support of optional values for double fields.

Bug: 1354063
Change-Id: Ifec1a076a8562f8008c8274bce56f90983fb33ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3840687


Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Claudio DeSouza <cdesouza@igalia.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040907}