Makefile: T3165: do not allow empty node.def files for op-mode commands
Commit 99440fc0 ("Makefile: fix logic to detect empty "node.def" files")
disabled the detection of empty node.def files for op-mode commands. The
generation of a duplicate and thus empty node.def file is not prohibited
by commit 17b5ac14 ("T3165: op-mode: prevent override of populated node.def file
with empty content") and thus the check is re-enabled!
(cherry picked from commit e99cdf40c72dec8b9019eca728aaad0f82c6030b)