doc/em/app.codes.nr: fix "139: no real data" error under GNU tbl

[tkchia]

Before the patch, running the GNU tbl(1) preprocessor on the troff file app.codes.nr would give a warning

$ tbl app.codes.nr >app.codes.nr.2
tbl:app.codes.nr:139: no real data
tbl:app.codes.nr:139: giving up on this table

and the table of suffixes

.z	no arguments
.l	16-bit argument
...	...

in the EM documentation would be left out of the tbl output.

[davidgiven]

Thanks!