Fix an exception while getting watchers for issue.
Fix an exception while getting watchers for issue.

Fix an exception while getting watchers if the author's account is
removed, by making User.getAuthor() returns anonymous if the user does
not exist.

ihoneymon/yobi,ChangsungKim/TestRepository01,bloodybear/yona,doortts/forked-for-history,brainagenet/yobi,ahb0327/yobi,brainagenet/yobi,doortts/fork-yobi,oolso/yobi,ihoneymon/yobi,naver/yobi,doortts/fork-yobi,violetag/demo,brainagenet/yobi,ahb0327/yobi,bloodybear/yona,naver/yobi,bloodybear/yona,ChangsungKim/TestRepository01,oolso/yobi,oolso/yobi,bloodybear/yona,doortts/fork-yobi,doortts/fork-yobi,yona-projects/yona,ahb0327/yobi,Limseunghwan/oss,violetag/demo,naver/yobi,ihoneymon/yobi,doortts/forked-for-history,yona-projects/yona,doortts/forked-for-history,yona-projects/yona,yona-projects/yona,Limseunghwan/oss,Limseunghwan/oss