Fix borg-list empty format

Should produce empty output, not default output.

src/borg/archiver.py

@@ -793,7 +793,7 @@ class Archiver:
                 archive = Archive(repository, key, manifest, args.location.archive, cache=cache,
                                   consider_part_files=args.consider_part_files)
 
-                if args.format:
+                if args.format is not None:
                     format = args.format
                 elif args.short:
                     format = "{path}{NL}"
@@ -804,7 +804,7 @@ class Archiver:
                 for item in archive.iter_items(lambda item: matcher.match(item.path)):
                     write(safe_encode(formatter.format_item(item)))
         else:
-            if args.format:
+            if args.format is not None:
                 format = args.format
             elif args.short:
                 format = "{archive}{NL}"