-Wformat-zero-length

I was adding gcc printf attributes to more functions in obscure places,
and now I'm seeing this in pg_upgrade:

relfilenode.c:72:2: warning: zero-length gnu_printf format string [-Wformat-zero-length]

So the options I can see are either adding the compiler option
-Wno-format-zero-length (with configure check and all), or hack up the
code to do something like this instead: Before:

prep_status("");

After:

prep_status("%s", "");

Comments?