[Rd] Non-invisible returns from title and mtext (PR#7397)

[p.dalgaard at biostat.ku.dk]

A small cosmetic item: In current versions we have

> plot(0)
> title("Foobar")
NULL

in which the printing of NULL didn't happen in 1.9.1. Same story with
mtext(), and both pretty apparent in the output from demo(graphics).

I can't offhand see what caused the change. 

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907