Groupmap doesn't work with empty or wildcard source group

[jdito]

Hi, recently updated from 3.2.3 to 3.2.7 and noticed that groupmap doesn't appear to work when either no source group or a wildcard source group is passed. I believe from the docs that this should work, and it did work in 3.2.3. Also downloaded the 3.3.0 tarball, and the behavior is the same there.

jditommasso@jditommasso1:~/scratch/rsync-3.3.0 [last runtime: 0s]
$ ls -l test-src/ test-dest-*
test-dest-explicit:
total 0K

test-dest-wildcard:
total 0K

test-src/:
total 24K
-rw-r--r-- 1 jditommasso users 0 Jul 17 15:00 test
jditommasso@jditommasso1:~/scratch/rsync-3.3.0 [last runtime: 0s]
$ id | awk -F, '{print $1","$2}'
uid=1861(jditommasso) gid=100(users) groups=100(users),20(dialout)
jditommasso@jditommasso1:~/scratch/rsync-3.3.0 [last runtime: 0s]
$ ./rsync --groupmap=users:dialout test-src/test test-dest-explicit/
jditommasso@jditommasso1:~/scratch/rsync-3.3.0 [last runtime: 0s]
$ ./rsync --groupmap=:dialout test-src/test test-dest-wildcard/
jditommasso@jditommasso1:~/scratch/rsync-3.3.0 [last runtime: 0s]
$ ls -l test-src/ test-dest-*
test-dest-explicit:
total 24K
-rw-r--r-- 1 jditommasso dialout 0 Jul 17 15:16 test

test-dest-wildcard:
total 24K
-rw-r--r-- 1 jditommasso users 0 Jul 17 15:16 test

test-src/:
total 24K
-rw-r--r-- 1 jditommasso users 0 Jul 17 15:00 test
jditommasso@jditommasso1:~/scratch/rsync-3.3.0 [last runtime: 0s]
$

Let me know if I'm misreading the docs or otherwise doing something wrong here.