wpseo_opengraph_image should run (even) if there's no image set

[monbauza]

Moved from https://yoast.atlassian.net/browse/IM-2033
Reporter: @jonoalderson

Our wpseo_opengraph_image (and wpseo_schema_main_image) filter(s) are used to customize the og:image output.

However, in the case that a post doesn’t have an og:image already set*, then this filter doesn’t run. Our documentation would imply that this is expected/correct behaviour, but this makes it impossible to (efficiently, sensibly) programmatically set images in many cases, and prevents advanced users from customizing the behaviour of their sites.

This is a long-standing issue, affecting many users (including me!); see #15721 and linked discussions.

I think we should treat it like a bug, and ‘correct’ the behaviour.

*Where ‘set’ includes fallback scenarios such as the presence of a featured post image, or post-level/site-level default.

[monbauza]

Related: #2416