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 [] [AlignDefault,AlignDefault] [5.555555555555555e-2,5.555555555555555e-2]

 [[]

 ,[]]

 [[[Plain [Str "1"]]

  ,[Plain [Str "2"]]]

 ,[[]

  ,[]]]]

```

```

% pandoc -f native -t rst

[Table [] [AlignDefault,AlignDefault] [5.555555555555555e-2,5.555555555555555e-2]

 [[]

 ,[]]

 [[[Para [Str "1"]]

  ,[Para [Str "2"]]]

 ,[[]

  ,[]]]]

^D

+---+---+

| 1 | 2 |

+---+---+

|   |   |

+---+---+

```