[iOS] Ensure that the received message has a value before sending it

Previously, when the received message does not have a cover value, it
resulted with a crash as the code try to access a value of an object
that did not exist. This CL prevent that by verifying if the object have
a cover value before or not.

Fixed: 324760726
Change-Id: Idb46701fcaf91670b9a5d21846e1fafd559e6e41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5285216
Commit-Queue: Ali Juma <ajuma@chromium.org>
Auto-Submit: Aliona Dangla <alionadangla@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1259160}