Cannot choose Admin Export options anymore

How to reproduce

Expected behaviour:

1. Go to the Administration
2. Click on the "export" link to access the "export section"
3. The export section is a form where we can set some options about the export (the name of the XAR, if the history is exported, etc...)
   
4. In that section, click on "export"
5. You get the XAR

What is happening now:

1. Go to the Administration
2. Click on the "export" link to access the "export section"
3. You get the XAR directly

So you cannot set the advanced export options, but you can still export the wiki.

Workaround

Use the URL /xwiki/bin/admin/XWiki/XWikiPreferences?editor=globaladmin&section=Export

Explanations

Regression caused by XWIKI-12607.

Admin Export section lead to /export/ action which used to display a form when no name parameter was provided but since XWIKI-12607 export action always export something and never display export.vm anymore.