Fix forking an empty repository (#6637) (#6653)

Fixes #6633

Signed-off-by: Segev Finer <segev@codeocean.com>

models/repo.go

@@ -2432,6 +2432,7 @@ func ForkRepository(doer, u *User, oldRepo *Repository, name, desc string) (_ *R
 		Description:   desc,
 		DefaultBranch: oldRepo.DefaultBranch,
 		IsPrivate:     oldRepo.IsPrivate,
+		IsEmpty:       oldRepo.IsEmpty,
 		IsFork:        true,
 		ForkID:        oldRepo.ID,
 	}