Media views EmbedLink should check for empty url string

[afercia]

Splitting this out from #29476. Noticed one small thing: embedding a URL in the "Insert from URL" field should handle also the case when users empty the field. To reproduce;

Edit/create a post then go in Add Media > Insert from URL

• paste an embeddable URL
• then empty the field
• at this point url is an empty string
• the AJAX call fires
• the "Link Text" field appears

You will also see a PHP notice in your console:

<b>Notice</b>:  Undefined variable: url in <b>.../ajax-actions.php</b> on line <b>2749

looks like this check in this line in /wp-includes/js/media/views/embed/link.js

if ( url && url.length < 6 ) {
  ...

should also check when url is falsey.

Also, not sure why 6 characters :) http:// is made by 7 characters but maybe I'm missing something.

[afercia]

My bad :) looks like the PHP notice was just in trunk and r32258 fixed it. Would recommend to still consider this ticket for the useless AJAX call to avoid and the "Link Text" field that shouldn't appear when emptying the URL field.

[wonderboymusic]

In 32463:

In EmbedLink, a URL needs to be at least 11 chars to be useful.

Fixes #32059.