`debusine collection create` doesn't read data input
I ran this against debusine.debian.net using 73ae327f0b557bd6d1413173d35c0cd9608eab83:
```console
$ 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.
issue