LaTeX writer: allow tables with empty cells to count as "plain."

This addresses a problem of too-wide tables when empty cells
are used.

Thanks to Joost Kremers for reporting the issue.

src/Text/Pandoc/Writers/LaTeX.hs

@@ -640,6 +640,7 @@ tableRowToLaTeX header aligns widths cols = do
   let scaleFactor = 0.97 ** fromIntegral (length aligns)
   let isSimple [Plain _] = True
       isSimple [Para  _] = True
+      isSimple []        = True
       isSimple _         = False
   -- simple tables have to have simple cells:
   let widths' = if not (all isSimple cols)