Jetpack Site Logo: add integer check for custom_logo

[fullofcaffeine]

Summary:
Resolves 53517-gh-wp-calypso and supports #19654

Implementation by @creativecoder.

Code recently added to Gutenberg syncs site_logo options and custom_logo theme mod. This causes a conflict with the legacy Jetpack site logo functionality.

This is an initial fix--we'll need to follow up to reconcile the new site_logo option and Jetpacks legacy logo functionality.

Related convo: p1623174912090300-slack-CBTN58FTJ.

Test Plan:

• Sandbox a Simple Site
• Activate a theme that uses jetpack_the_site_logo to display a logo, such as premium/soho (wp theme activate premium/soho --url=[private link])
• Navigate to the customizer, set a logo, and save the changes
• Without this change, the logo does not display
• With this change, the logo should display

Reviewers: paulbunkham, kraftbj, fullofcaffeine, bernhardreiter

Tags: #touches_jetpack_files

Differential Revision: D62504-code

This commit syncs r226957-wpcom.

Fixes #

Changes proposed in this Pull Request:

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Go to '..'

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Jetpack plugin:

• Next scheduled release: July 6, 2021.
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