Describe the bug
When trying to create a new page in the panel, if the "URL appendix" field is empty (either because the slug rules leave it empty or you manually clear it), clicking the "Create draft" button shows this error: "A page draft with the URL appendix "" already exists"

To Reproduce
Steps to reproduce the behavior:

1. Click "Add" button in panel
2. Type in a title
3. Clear out the "URL appendix" field
4. Click "Create draft"

Expected behavior
Should say that a URL appendix is required

Screenshots

Kirby Version
3.2.5 and 3.3.0-rc.1

Console output

[Error] Failed to load resource: the server responded with a status of 400 (Bad Request) (children, line 0)
[Error] {status: "error", message: "A page draft with the URL appendix \"\" already exists", code: 400, exception: "Kirby\\Exception\\DuplicateException", key: "error.page.draft.duplicate", …}
	(anonymous function) (app.js:1:260019)
	(anonymous function) (app.js:1:255723)
	promiseReactionJob

Desktop (please complete the following information):

• OS: macOS 10.15
• Browser: Safari
• Version: 13.0.2