When enabling all syntaxes, only XWiki/2.1 is listed in the admin

To reproduce:

• go to http://127.0.0.1:8080/xwiki/bin/admin/XWiki/XWikiPreferences?editor=globaladmin&section=Syntaxes&space=XWiki
• uncheck everything but one syntax
• save -> all the unchecked syntaxes are listed in the "Default page syntax" combo box
• uncheck the last remaining syntax
• save -> only XWiki 2.1 can now be chosen