Allow existing empty dir when migrating files

Remove old empty directory if it exists - otherwise, we move old/data to
new/data/data.

qutebrowser/utils/standarddir.py

@@ -359,9 +359,11 @@ def _move_data(old, new):
     log.init.debug("Migrating data from {} to {}".format(old, new))
 
     if os.path.exists(new):
-        message.error("Failed to move data from {} as {} already exists!"
-                      .format(old, new))
-        return False
+        if os.listdir(new):
+            message.error("Failed to move data from {} as {} is non-empty!"
+                          .format(old, new))
+            return False
+        os.rmdir(new)
 
     try:
         shutil.move(old, new)

tests/unit/utils/test_standarddir.py

@@ -389,7 +389,7 @@ class TestDataMigrations:
             standarddir._move_webengine_data()
 
         record = caplog.records[-1]
-        expected = "Failed to move data from {} as {} already exists!".format(
+        expected = "Failed to move data from {} as {} is non-empty!".format(
             old_path, new_path)
         assert record.message == expected