debusine collection create doesn't read data input

I ran this against debusine.debian.net using 73ae327f:

$ python3 -m debusine.client collection create debian:qa-results sid

                       ID:   #1424
                     Name:   sid
                 Category:   debian:qa-results
   Full history retention:   unlimited
  Metadata only retention:   unlimited
                     Data:   {}

Despite python3 -m debusine.client collection create --help saying that --data defaults to stdin, to my surprise this did not prompt for any data on stdin.