Warning: Undefined array key "back_to_top_speed" in back_to_top_page_attachments()

Problem/Motivation

When using the Back To Top module, the following warning appears:
Warning: Undefined array key "back_to_top_speed" in back_to_top_page_attachments() (line 35 of modules/contrib/back_to_top/back_to_top.module).

The warning occurs when the back_to_top_speed value is not set in the module's settings.

Steps to reproduce

1. Install and enable the "Back To Top" module.
2. Leave the "back_to_top_speed" configuration empty or unset.
3. Load any page where the module is active.
4. Check the logs or observe the warning on the page.

Proposed resolution

To prevent the warning, the value of back_to_top_speed should be checked before assigning it.

Replace:

$attachments['#attached']['drupalSettings']['back_to_top']['back_to_top_speed'] = $settings['back_to_top_speed'];
With:
$attachments['#attached']['drupalSettings']['back_to_top']['back_to_top_speed'] = isset($settings['back_to_top_speed']) ? $settings['back_to_top_speed'] : 1000;

This ensures that a default value (1000) is used if back_to_top_speed is not set.

Remaining tasks

Apply the proposed fix to the codebase.
Test the module after the fix to confirm that the warning no longer appears.