LifterLMS Navigation Link Block doesn't work properly when the default link is used

[eri-trabiccolo]

Reproduction Steps

• using the Site Editor add a LifterLMS Link block to the menu
• Do not modify the link selection: the Dashboard should be selected by default
• Save and check the front-end

Expected Behavior

• I would expect to see the Dashboard menu item

Actual Behavior

• No Dashboard menu item

I think we can safely consider the attributes 'label' = 'Dashboard' and 'page' = 'dashboard' when they're not set (yet).
Workaround:
select any page, then select the Dashboard one and save.

Error Messages / Logs

[20-Jun-2023 06:58:33 UTC] PHP Warning:  Undefined array key "page" in /Users/rock/Dev/plugins/lifterlms/includes/class.llms.nav.menus.php on line 383

System and Environment Information

LifterLMS 7.2.1

This issue has be recreated:

• Locally
• On a staging site
• On a production website
• With only LifterLMS and a default theme

Browser, Device, and Operating System Information

n/a