Fix a regression in the previous release that could result in empty author folders remaining in the library when the author of a book is changed

src/calibre/db/backend.py

@@ -1872,6 +1872,9 @@ class DB:
 
         if os.path.exists(spath):
             copy_tree(os.path.abspath(spath), tpath, delete_source=True, transform_destination_filename=transform_format_filenames)
+            parent = os.path.dirname(spath)
+            with suppress(OSError):
+                os.rmdir(parent)  # remove empty parent directory
         else:
             os.makedirs(tpath)
         update_paths_in_db()

src/calibre/db/tests/filesystem.py

@@ -127,7 +127,10 @@ class FilesystemTest(BaseTest):
         from calibre.ebooks.metadata.book.base import Metadata
         cache.set_metadata(1, Metadata('t1', ('a1', 'a2')))
         check_that_filesystem_and_db_entries_match(1)
-
+        # check that empty author folders are removed
+        for x in os.scandir(cache.backend.library_path):
+            if x.is_dir():
+                self.assertTrue(os.listdir(x.path))
 
     @unittest.skipUnless(iswindows, 'Windows only')
     def test_windows_atomic_move(self):