delete-trailing-whitespace removes empty LHS code lines

In `literate-haskell-mode`, given some input like

```
foo
> bar
>
> baz
quux
```

The command `delete-trailing-whitespace` removes the empty code lines, producing:

```
foo
> bar

> baz
quux
```

The problem with this is that this results in two separate code blocks for `bar` and `baz` which are potentially typeset very differently than a single code block containing three lines with the middle being empty.

Reproduce the example in literate-haskell-mode by running delete-trailing-whitespace on the supplied buffer. Trace the command's handling of the empty line prefixed with `>`, and consider the issue complete when that line remains part of the surrounding code block instead of becoming a separate block.

Written by the indexing model from the issue text.