Fix lint

Signed-off-by: Elliot Gunton <elliotgunton@gmail.com>

src/hera/workflows/_meta_mixins.py

@@ -561,13 +561,14 @@ def _create_subnode(
         if _context.pieces[0] != self and isinstance(self, WorkflowTemplate):
             # Using None for cluster_scope means it won't appear in the YAML spec (saving some bytes),
             # as cluster_scope=False is the default value
+            assert isinstance(template, Template)
             template_ref = TemplateRef(
                 name=self.name,
                 template=template.name,
                 cluster_scope=True if isinstance(self, ClusterWorkflowTemplate) else None,
             )
             # Set template to None as it cannot be set alongside template_ref
-            template = None
+            template = None  # type: ignore
 
         _context.declaring = False
         if isinstance(_context.pieces[-1], (Steps, Parallel)):