fix PHP warnings about non-string

[cliffordp]

Stops the generation of these WP_DEBUG messages (https://gist.github.com/cliffordp/42b277d0321c78f59cd2d57faeb0a93d) with the Community Events plugin by Modern Tribe (https://theeventscalendar.com/product/wordpress-community-events/) when Shortcode Embeds is active (https://central.tri.be/issues/72412) and Community Events reCAPTCHA is in use (e.g. anonymous submissions)

Fixes #

Changes proposed in this Pull Request:

Testing instructions:

Proposed changelog entry for your changes:

---

• Make sure your changes respect WordPress' coding standards.

[georgestephanis]

I'm somewhat careful on just short circuiting when input isn't what is expected, fearing we may let something concerning through (at first glance anyways) -- can you give a quick example for why $html wouldn't be a string in this instance, and what we can do to more safely handle that use case?

[georgestephanis]

I don't think it's necessary to include the second is_string() here -- $element is the key from the array, so there shouldn't be any issues with it.

[cliffordp]

@georgestephanis, I added

var_dump($html);
var_dump($element);

and got a lot of these:

• 
• 
• 
• and many more

So it looks like you could drop || ! is_string( $element )

I'm not really sure where these empty arrays for $html are coming from.

Here's a copy of the latest version of Community Events (CE) add-on for https://wordpress.org/plugins/the-events-calendar/ --
the-events-calendar-community-events.4.4.1.zip

Here's how to reproduce the issue:

• Enable Jetpack 4.5's Shortcode Embeds: https://cl.ly/1p3S2a3o3q02
• Make sure reCAPTCHA and Anonymous submissions are enabled
• Try to create a new event via CE form (error happens when you click Submit)

[georgestephanis]

I'm talking to @cliffordp in .org Slack right now -- this isn't a Jetpack problem, it looks like it's a case of another plugin passing an array instead of a string to wp_kses_post()

Closing as it's not ours to fix.

[cliffordp]

Thanks so much for your help, @georgestephanis!!!