delete-trailing-whitespace removes empty LHS code lines

[gergoerdi]

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.