Requested function: just-one-empty-line()

[Nordlöw]

I'm looking for a function just-one-empty-line(), or empty-lines() for
the multi-line-variant, that does kind of what just-one-space() does
but instead works on empty lines. Any suggestions?

Example:

pre;\n
\n
\n
post;

is converted to

pre;\n
\n
post;

Thanks in advance,
Nordlöw