Ad block: bail early when we do not have any ad params.

[jeherve]

Fixes #13453

Changes proposed in this Pull Request:

• In some cases, we may not have any ad parameters. When that's the case, bail early to avoid fatals (see Ad block causes posts list not to load in Calypso #13453).

Testing instructions:

• Start with a site on a Premium or Professional plan.
• Activate the Ads module
• Create a new post, and insert an ad block.
• Publish a post.
• Load your site's posts in Calypso: https://wordpress.com/posts/
• Watch the fatal in your logs. This happens because the WordAds Object returned when loading the posts looks like this:

(
    [params] =>
    [ads] => Array
        (
            [0] => Array
                (
                    [location] => gutenberg
                    [width] => 300
                    [height] => 250
                )

        )

)

• Apply patch.
• Notice the fatal does not appear anymore.

Proposed changelog entry for your changes:

• Ads Block: avoid PHP errors when loading posts via the WordPress.com interface.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello jeherve! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D32727-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: October 1, 2019.
Scheduled code freeze: September 24, 2019

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