ob-gnuplot: Honour :missing argument

* lisp/ob-gnuplot.el (org-babel-gnuplot-table-to-data): Force applying
`org-babel-gnuplot-quote-tsv-field' to empty table cells when
converting org tables to gnuplot input.  By default,
`orgtbl-to-generic' ignores :fmt for empty table cells.  We need to
set :raw to non-nil AND :backend to non-nil to force custom formatting
of empty table cells.

Fixes https://orgmode.org/list/20210617211347.59e4de56@happy.intern.roklein.de

lisp/ob-gnuplot.el

@@ -290,7 +290,7 @@ Pass PARAMS through to `orgtbl-to-generic' when exporting TABLE."
 	      (orgtbl-to-generic
 	       table
 	       (org-combine-plists
-		'(:sep "\t" :fmt org-babel-gnuplot-quote-tsv-field)
+		'(:sep "\t" :fmt org-babel-gnuplot-quote-tsv-field :raw t :backend ascii)
 		params)))))
   data-file)