The Syntax Conversion dialog is displayed when changing the syntax of a page that has nothing to convert

Steps to reproduce

1. Create a new page (for example with the "Create" button) and open it in the default (WYSIWYG) editor, without typing anything.
2. In the "Page syntax" field of the editor, select another syntax, for example "CommonMark Markdown 1.2".

Result

The "Syntax Conversion" dialog opens and asks: "Do you want to also convert the page content and meta data from the previous XWiki 2.1 syntax to the selected CommonMark Markdown 1.2 syntax? Choosing 'No' will only change the syntax identifier, without modifying the page content." The user has to answer a question about converting content that doesn't exist, and "Yes" is the highlighted default.

Expected

When there is nothing to convert, meaning the page content is empty and the page has no textarea xproperty containing wiki markup, changing the syntax should only change the syntax identifier, without showing the dialog.

Notes

• Reproduced on XWiki 18.7.0 (CKEditor and the wiki editor, on a new unsaved page).
• syntaxPicker.js (flamingo skin resources) calls maybeAskForSyntaxConversionConfirmation() on every syntax change. The only emptiness check (contentField.val()) is done afterwards, in the wiki editor's conversion handler, once the user has already answered.