not overwrite with empty cells

hello,

(I don't know if it's the right place to as ; sorry if not)
in libreoffice calc, when I import use the import data, it pastes 
automatically on existing cells on the sheet, which is a good point, 
though it overwrite on cells even if the pasted cell is empty ; I would 
like to overwrite existing cell only if the pasted cell is not empty
it's a bit the way it goes when pasting special
I am 95% sure it was going this way on the previous versions though it 
changed on a recent update (or I changed any option without noticing)
The only way I found to do so is to paste on another sheet, then copy 
those cells and then special past them on the right place.
A bit complicated though ; any way to make it simpler?
Thanks
Ben