tmpfiles: Make C still copy if the destination directory is empty

Fixes #11287.
diff --git a/src/tmpfiles/tmpfiles.c b/src/tmpfiles/tmpfiles.c
index 19225f8..03a9e1d 100644
--- a/src/tmpfiles/tmpfiles.c
+++ b/src/tmpfiles/tmpfiles.c
@@ -1463,7 +1463,7 @@
                          dfd, bn,
                          i->uid_set ? i->uid : UID_INVALID,
                          i->gid_set ? i->gid : GID_INVALID,
-                         COPY_REFLINK);
+                         COPY_REFLINK | COPY_MERGE_EMPTY);
         if (r < 0) {
                 struct stat a, b;