MDL-48288 gradebook: fix floating footer exception when averages disabled

The code we use to update styles on floating elements did not account for
situations where those floating elements don't exist. This was discovered
when a test site didn't have column averages enabled, resulting in no
floating footer element being created, so when _handleScrollEvent was
fired the first time, a null reference exception was thrown trying to
apply styles to a non-existent element.