Do not use empty() on \OC\Group\Group objects in \OC\Group\MetaData::fetchGroups().

empty() should not be used on \OC\Group\Group objects, because empty() checks
if there are users in the group. We do want to check for the existance of
the group, ie. whether GroupManager::get() returned a group or not, instead.

Additionally, $grp was renamed to $group inside
 \OC\Group\MetaData::fetchGroups().

Signed-off-by: Stephan Peijnik <speijnik@anexia-it.com>