Fixed a complaint from ruff check

sqlite_utils/cli.py

@@ -2891,11 +2891,16 @@ def convert(
         raise click.ClickException(str(e))
     if dry_run:
         # Pull first 20 values for first column and preview them
-        preview = lambda v: fn(v) if v else v  # noqa: E731
         if multi:
-            preview = (
-                lambda v: json.dumps(fn(v), default=repr) if v else v
-            )  # noqa: E731
+
+            def preview(v):
+                return json.dumps(fn(v), default=repr) if v else v
+
+        else:
+
+            def preview(v):
+                return fn(v) if v else v
+
         db.conn.create_function("preview_transform", 1, preview)
         sql = """
             select