Correctly handle empty row:
```
% pandoc -f markdown -t rst
+---+---+
| 1 | 2 |
+---+---+
|   |   |
+---+---+
^D
+---+---+
| 1 | 2 |
+---+---+
|   |   |
+---+---+
```

Temporarily added these to figure out what is happening
on Windows builds.
```
% pandoc -f markdown -t native
+---+---+
| 1 | 2 |
+---+---+
|   |   |
+---+---+
^D
[ Table
  ( "", [], [] )
  ( Caption Nothing [] )
  [
    ( AlignDefault, ColWidth 5.555555555555555 e- 2 )
  ,
    ( AlignDefault, ColWidth 5.555555555555555 e- 2 )
  ]
  ( TableHead ( "", [], [] ) [] )
  [ TableBody
    ( "", [], [] )
    ( RowHeadColumns 0 ) []
    [ Row
      ( "", [], [] )
      [ Cell
        ( "", [], [] ) AlignDefault
        ( RowSpan 1 )
        ( ColSpan 1 )
        [ Plain [ Str "1" ] ]
      , Cell
        ( "", [], [] ) AlignDefault
        ( RowSpan 1 )
        ( ColSpan 1 )
        [ Plain [ Str "2" ] ]
      ]
    , Row
      ( "", [], [] )
      [ Cell
        ( "", [], [] ) AlignDefault
        ( RowSpan 1 )
        ( ColSpan 1 ) []
      , Cell
        ( "", [], [] ) AlignDefault
        ( RowSpan 1 )
        ( ColSpan 1 ) []
      ]
    ]
  ]
  ( TableFoot ( "", [], [] ) [] )
]
```

```
% pandoc -f native -t rst
[Table ("",[],[]) (Caption Nothing
 [])
 [(AlignDefault,ColWidth 5.555555555555555e-2)
 ,(AlignDefault,ColWidth 5.555555555555555e-2)]
 (TableHead ("",[],[])
 [Row ("",[],[])
  [Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
   []
  ,Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
   []]])
 [(TableBody ("",[],[]) (RowHeadColumns 0)
  []
  [Row ("",[],[])
   [Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
    [Para [Str "1"]]
   ,Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
    [Para [Str "2"]]]
  ,Row ("",[],[])
   [Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
    []
   ,Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
    []]])]
 (TableFoot ("",[],[])
 [])]
^D
+---+---+
| 1 | 2 |
+---+---+
|   |   |
+---+---+
```