Attributes do not save on the default translation

Issue information

From user: recrit

Contribution record

Fork management

Problem/Motivation

This is a regression from the combination of #3212105: Class values in menu link submit hook are appended instead of replaced and #3196822: Menu link attributes seems to break menu item translation that are in 8.x-1.3-rc1.
- When saving the default translation with NEW link attributes, the link attributes will not save.
- The following warning is thrown:

Warning: Undefined variable $menu_link_attributes in menu_link_attributes_menu_link_content_form_entity_builder() (line 190 of modules/contrib/menu_link_attributes/menu_link_attributes.module).
Edited by drupalbot
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