Potential PHP notice in `wp_ajax_wp_privacy_erase_personal_data()`

[desrosj]

The $eraser_key variable is passed to the wp_privacy_personal_data_erasure_page filter, but it is only set if there is one or more data erasers defined.

The variable should be initialized earlier to avoid a PHP notice.

[allendav]

To test:

• WP_DEBUG true
• Comment out add_filter( 'wp_privacy_personal_data_erasers', 'wp_register_comment_personal_data_eraser' ); in wp-includes/default-filters.php
• Make sure you have NO other plugins installed that contribute erasers - the goal is to get to 0 erasers :P
• Perform an erasure on an uncompleted request. Ensure it completes and is marked as such.
• Ensure no notices in the logs

[allendav]

Updated patch - accidentally included default-filters test hack.

Also fixed same flaw which also exists in the exporters logic.

[SergeyBiryukov]

In 43228:

Privacy: Avoid a PHP notice in wp_ajax_wp_privacy_erase_personal_data(), make sure $eraser_key is always defined.

Props allendav.
Fixes #44040.

[SergeyBiryukov]

In 43229:

Privacy: Avoid a PHP notice in wp_ajax_wp_privacy_erase_personal_data(), make sure $eraser_key is always defined.

Props allendav.
Merges [43228] to the 4.9 branch.
Fixes #44040.

[desrosj]

Moving to the new Privacy component.