GetFeeds must always discard actions with dangling repo_id (#19598) (#19629)

Co-authored-by: Loïc Dachary <loic@dachary.org>
(cherry picked from commit b536b65189)

Conflicts:
        models/action_test.go
	  The GetFeeds function does not have a Context argument in 1.16.
	models/action.go
	  The SQL statement is essentially the same in 1.16 but
	  structured differently. The Join() was copied and the
   	  created_unix field prefixed with `action`.
	models/action_list.go
	  in 1.16 the loadRepoOwner method did not exist and
	  it was done in the RetrieveFeeds method of web/feed/profile.go.
          The safeguard to skip when act.Repo == nil was moved there.

routers/web/feed/profile.go

@@ -34,6 +34,9 @@ func RetrieveFeeds(ctx *context.Context, options models.GetFeedsOptions) []*mode
 	}
 
 	for _, act := range actions {
+		if act.Repo == nil {
+			continue
+		}
 		repoOwner, ok := userCache[act.Repo.OwnerID]
 		if !ok {
 			repoOwner, err = user_model.GetUserByID(act.Repo.OwnerID)