runtime(netrw): restore blank line cleanup after file listing

runtime(netrw): restore blank line cleanup after file listing

Problem:  v182 refactoring removed blank line cleanup (g/^$/d) from
          s:LocalListing(), causing empty lines between directories
          and files.
Solution: Add the missing cleanup after append() in s:PerformListing()
          (uma-chan).

closes: #17672

Co-authored-by: Luca Saccarola <96259932+saccarosium@users.noreply.github.com>
Signed-off-by: uma-chan <127664533+i9wa4@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>