Missing checks causing notices

Problem/Motivation

When certain values like 'below' are not set notices are thrown.
- Notice: Undefined index: below in translatable_menu_link_uri_iterate_menu()
- Notice: Undefined index: original_link in translatable_menu_link_uri_iterate_menu()
- Notice: Undefined variable: plugin_id in translatable_menu_link_uri_iterate_menu

Steps to reproduce

Create menu items without a sublevel.

Not having any items below the first will generate the first notice.
Original link can be empty so that can throw a notive.
If that happens $plugin_id is never set and has no default value which will throw another notice.

Proposed resolution

Perform checks and provide a default value where necessary.

I have attached a patch for this.