400 Bad Request exception occurs when saving content item with empty non-mandatory URL but with Text.

Prereq.: Create a Content Type with a URL field type which not required.

Steps

1. Go to Content / Content structure.
2. Start creating new item using the CT with URL.
3. Insert both URL and Text. (screen1)
4. Save. [OK]
5. Remove both URL and Text, leave them blank. (screen2)
6. Save. [OK]
7. Insert only URL. (screen3)
8. Save. [OK]
9. Insert only Text. (screen4)
10. Save.

Actual: 400 Bad Request "Argument '%argumentName%' is invalid: expected value to be of type 'string'" (screen5)

Remarks: I guess workaround is to make URL required.