Some static lists in XWiki.XWikiPreferences have the "---" value

In XWiki.XWikiPreferencesInitalizer ( https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/internal/mandatory/XWikiPreferencesDocumentInitializer.java#L106 ), we have some lines like this:

needsUpdate |= bclass.addStaticListField("editor", "Default Editor", "---|Text|Wysiwyg");
needsUpdate |= bclass.addStaticListField("registration_anonymous", "Anonymous", "---|Image|Text");
needsUpdate |= bclass.addStaticListField("leftPanelsWidth", "Width of the left panel column",
                "---|Small|Medium|Large");

The problem is that the value "--- " should not be stored in the db, since it means "default". Instead, the default should be the empty value with a displayer to show "--- " in the UI.

The problem is that xwiki.getSpacePreference() does not fallback to xwiki.getWikiPreference() when the value is "--- ", because it currently only fallback on empty value (ex: XWIKI-10787).

Different fixes are possibles:

1. change XWiki.XWikiPreferences to not store "--- " anymore with either a migrator to remove the current "--- " values or either a fallback on xwiki.getSpacePreference() when the value is "--- " (for retro-compatibility)
2. only change xwiki.getSpacePreference() to fallback when the value is "--- ".