Properly format empty nodes on their own lines

## Original XML

``` xml
<book>
  <title>See Spot Run</title>
  <year>1942</year>
  <color></color>
  <weight>   </weight>
  <pages>102</pages>
</book>
```

## Without patch

Notice "weight" and "pages" on the same line

``` haml
%book
  %title See Spot Run
  %year 1942
  %color
  %weight   %pages 102
```
## With patch

Notice "weight" and "pages" cleaned up

``` haml 
 %book
    %title See Spot Run
    %year 1942
    %color
    %weight
    %pages 102
```