Don't create Menu UIX components without an id

In XWiki 9.8RC1 Thomas added some warning to discover when some UIX component are created with without an id and to log a warning when that's the case. This made us discover that the Menu app does this and it looks like hack that would need fixing (Note that FTM we removed the warning, see XWIKI-14707).

Also note that we need a proper way to disable a UIX component, see XWIKI-13076

Reproduction steps

• go to the menu app http://localhost:8080/xwiki/bin/view/Menu/
• click on "add new entry", define an entry name and click on the "+" icon
• leave the Menu Display Location to Nowhere (picking another option will introduce a UIXP id in the produced XObject)
• save the page, the edit the page on object mode and check the values of the XWiki.UIExtensionClass XObject

Expected

The value of the Extension Point ID is not empty

Actual

The value of the Extension Point ID is empty

Analysis

It makes sense to have an empty UIXP id value since the menu entry is not expected to be displayed.
Options:

• insert a made up UIXP id specific to menu entries that should not be displayed
• store the values in another kind of object than XWiki.UIExtensionClass untils another value is selected for Menu Display Location
• insert an existing UIXP id with an ignore=true parameter (but depends on XWIKI-14707)