Posts: Fix detectImage post normalizer when there's an iframe with an empty src

[youknowriad]

closes #11411

The rule was failing silently on Chrome and was stoping the posts loading on Safari.

Testing instructions

• Create a post containing a getty embed ( [getty src="539147864" width="594" height="441"] )
• Navigate to /posts using Safari
• The posts list should load properly

[matticbot]

Test live: https://calypso.live/?branch=fix/getty-embed

[kriskarkoski]

I've confirmed this allows the posts list to load when it includes a Getty embed.

Is it expected that the embed does not render as the expected image or show the shortcode?

[youknowriad]

I can confirm that it's not the only embed not rendered on the post list (Getty is not even among the whitelisted iframe lists). Maybe more background from @timmyc?

[timmyc]

I don't really know the background on these normalizer rules, /cc @samouri who was the last one to work on this logic.

The changes here seem reasonable to me though.

[samouri]

Interesting. This wasn't broken for me on Chrome but I was able to repro in Safari. The fix worked and the code LGTM. 🚢

I do wonder why this is happening though. On Safari I was also seeing a second error that seemed related to Getty javascript within the iframe: . The error disappeared with this fix so I'm thinking they could be related. My gut says it has something to do with the iframe sandboxing