Add isset() check in case source type is absent

[westonruter]

Fixes #7862

The only behavioral change in this PR is 66ca42e, the addition of an isset() check. (However, it's not clear why the key isn't set in the first place.)

Otherwise, it just bumps the minimum version of WP from 6.3 to 6.5 due to Gutenberg increasing its minimum required version of WP. And it fixes a test breakage due to a CSS change in the Twenty Ten theme.

[github-actions]

Plugin builds for 296c233 are ready 🛎️!

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Checksums

# Development build checksums
605861f2cf452475ee2454e1ab1f78c247be2ab844a6bc1de424c3283c4ca45f *amp.zip

# Production build checksums
6c603ea79356b948a7590dd3bc912a249a6a2c060c3e0bb2786400542db818e1 *amp.zip

Warning

These builds are for testing purposes only and should not be used in production.

[westonruter]

Some of the tests are failing because Gutenberg is being installed on WP 6.3, but it requires WP 6.5+ now.

Other tests are failing (trivially) due to a CSS change.

[thelovekesh]

Also, need to config ig there is any redundant code added for backward compatibility for WP < 6.5. But that can be part of other PR if any.