LibreOffice Calc - Delete empty rows

Not in one go, but with a small workaround:

1. Select the whole of your spreadsheet (e.g. by clicking in the empty square to the left of the "A" column header).
2. Turn on Autofilter.
3. From the drop down list at the top of a column, select "Empty" in each column until you end up with only empty rows.
4. Select and delete these.
5. Reset the filter.

Your non-empty lines will reappear.

Another workaround would be to sort the data by a column that you know will contain a value for every single row that contains data.

1. Select column range
2. Data > Sort...

All relevant rows should end up at the top.