chown compatibility: `chown root: abc` doesn't change group

I noticed this today:

**gnu/coreutils:**
```
% mkdir abc
% stat -c%u:%g abc
1000:1000
% doas chown root: abc
% stat -c%u:%g abc
0:0
```

**uutils/coreutils:**
```
# mkdir abc
# chown 1000 abc
# chgrp 1000 abc
# stat -c%u:%g abc
1000:1000
# chown root: abc
# stat -c%u:%g abc
0:1000
```