Fix undefined index warnings in Latest Comments & Latest Posts

[noisysocks]

Fixes #12119.

In #12003, prepare_attributes_for_render was changed so that the $attributes array that is passed to render_callback contains the same keys as what is passed to the block on the front-end. That is, it omits keys that are not set.

This caused some 'Undefined index' notices in the Latest Comments and Latest Posts blocks. We need to be using isset() when using attributes that do not have a default value defined.

Testing

1. Ensure that PHP is set to report notices, e.g. using error_reporting( E_ALL );
2. Create a post
3. Insert a Latest Comments block
4. Insert a Latest Posts block
5. Preview the post
6. There should be no PHP notice messages

[pento]

Looks good, let's go with it.

[gziolo]

This caused some 'Undefined index' notices in the Latest Comments and Latest Posts blocks. We need to be using isset() when using attributes that do not have a default value defined.

Shouldn't it always be the case in PHP to do isset() check? 😅
At some point we will make attributes filterable so it might become only worse.

[noisysocks]

If the attribute definition has a 'default' value then it's safe to not do an isset() in the render_callback because prepare_attributes_for_render will take care of ensuring that that attribute is set.

The point about filters is interesting... I'm not sure about that. I'd say that it should be the responsibility of the developer who is adding the filter to ensure that they're not unsetting variables that are used by Core code 🙂