We need to initialize `schema` to `None`, otherwise it will be an access
to an uninitialized variable when looking up invalid schemata:
[...]
File "[...]/osbuild/meta.py", line 583, in get_schema
schema = Schema(schema, name or klass)
UnboundLocalError: local variable 'schema' referenced before assignment
Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>