OpenTable: Fix error when pasting embed codes

[pablinos]

We were accidentally relying on an error being thrown during the
onChange handler, when an embed code was pasted into the input. The
error meant that the event bubbled up and the onSubmit event handled the
input correctly.

Changes proposed in this Pull Request:

This change sets the saveTransform function to check if the current
input looks like an embed. If it does, then it prevents it from being
treated like a token, and the onChange handler is never called.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

This is a bug fix to an existing feature

Testing instructions:

Without this branch, and with the browser developer console open paste in an embed code into an OpenTable block - for example:

<script type='text/javascript' src='//www.opentable.com/widget/reservation/loader?rid=412810&type=standard&theme=standard&iframe=true&domain=com&lang=en-US&newtab=false&ot_source=Restaurant%20website'></script>

Hit Enter or click the Embed button.
Notice that you see an error along the lines of parsed is null

Do the same thing with this branch and you should not see the error.

Proposed changelog entry for your changes:

I'm not sure this requires a changelog entry, but if it does:

Bug fixed that caused an unhandled error when inserting an OpenTable embed code

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello pablinos! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D38920-code before merging this PR. Thank you!

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: March 3, 2020.
Scheduled code freeze: February 25, 2020

Generated by 🚫 dangerJS against a9ae9a2

[scruffian]

LGTM. Great catch!

[jeherve]

Tests well for me.

[scruffian]

r203005-wpcom