Fix for empty objects (#180)

[RomanTsegelskyi]

Fix for #180. Some example,

> mt <- mtcars[mtcars$mpg < 0, 1:4]
> res <- pander(mt)

-----------------------
 mpg   cyl   disp   hp 
----- ----- ------ ----
 NA    NA     NA    NA 
-----------------------

> colnames(mt) <- NULL
> res <- pander(mt)
Warning message:
In pandoc.table.return(...) :
  Object is empty and without header. No output will be produced
> mt <- matrix(0, nrow = 0, ncol = 5)
> res <- pander(mt)
Warning message:
In pandoc.table.return(...) :
  Object is empty and without header. No output will be produced
> colnames(mt) <- 1:5
> res <- pander(mt)

-------------------
 1   2   3   4   5 
--- --- --- --- ---
NA  NA  NA  NA  NA 
-------------------