meta: fix lookup of invalid schemata

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>

osbuild/meta.py

@@ -564,6 +564,7 @@ class Index:
         `None` and any validation will fail.
         """
         cached_schema: Optional[Schema] = self._schemata.get((klass, name, version))
+        schema = None
 
         if cached_schema is not None:
             return cached_schema