Use ghost users in issues/PRs (#1831)

models/issue_list.go

@@ -69,7 +69,10 @@ func (issues IssueList) loadPosters(e Engine) error {
 	}
 
 	for _, issue := range issues {
-		issue.Poster = posterMaps[issue.PosterID]
+		var ok bool
+		if issue.Poster, ok = posterMaps[issue.PosterID]; !ok {
+			issue.Poster = NewGhostUser()
+		}
 	}
 	return nil
 }
@@ -173,7 +176,10 @@ func (issues IssueList) loadAssignees(e Engine) error {
 	}
 
 	for _, issue := range issues {
-		issue.Assignee = assigneeMaps[issue.AssigneeID]
+		var ok bool
+		if issue.Assignee, ok = assigneeMaps[issue.AssigneeID]; !ok {
+			issue.Assignee = NewGhostUser()
+		}
 	}
 	return nil
 }