#388 Post-Python 3: Add Variant Title errors out on a non-existing title ID

[Ahasuerus]

"Add Variant Title" errors out on a non-existing title ID. The error can be recreated by loading a title into the "Add Variant Title" edit form, then deleting the title via a different Web page, the clicking "Submit Data". The line which reads "parent_data = SQLloadTitle(new.title_id)" in edit/submitvariant.py doesn't check if "parent_data" is empty.