print_group_picture prints group links even when there is no picture for the group, so the link is like <a ...></a> - this creates an invisible tabstop which is confusing for screenreader and other keyboard users, as well as wasting a few bytes.

To reproduce, do the following:

1) create course, add yourself as teacher
2) create 3 groups (these are all 3 possible cases with regard to group pictures):
a) with a picture
b) with a picture but with 'hide picture'
c) with no picture
3) create forum, start discussion

• observe that if you focus the user name at top of the post and press tab, it tabs through the 2 pictures but there is then an 'invisible' tab spot

• if you log in as a student, the 'hide picture' option does work, apparently to remove both the picture AND the link, i.e. a group with hide picture is better than one with no picture at all from that perspective. Presumably it is intentional that 'hide picture' does not work for users those with managegroups capability i.e. it really means 'hide picture from students'. (how DO you get rid of a group picture once you've made one? ho hum)

I propose making it so that if a group has no picture, then the output from print_group_picture for that group will be an empty string and not a link with empty content.