Fix mypy default args issues in Workflow (#918)

**Pull Request Checklist**
- [ ] Towards fixing #911
- [ ] Tests added
- [ ] Documentation/examples added
- [ ] [Good commit messages](https://cbea.ms/git-commit/) and/or PR
title

**Description of PR**
Fixes mypy linting issues with default args for mixin fields of
arguments, metrics, volumes in workflow.py

Signed-off-by: Max Shatkhin <[email protected]>

src/hera/workflows/workflow.py

@@ -248,12 +248,12 @@ def _build_templates(self) -> Optional[List[TTemplate]]:
     arguments: Annotated[
         ArgumentsT,
         _WorkflowModelMapper("spec.arguments", ArgumentsMixin._build_arguments),
-    ]
+    ] = None
     metrics: Annotated[
         MetricsT,
         _WorkflowModelMapper("spec.metrics", MetricsMixin._build_metrics),
-    ]
-    volumes: Annotated[VolumesT, _WorkflowModelMapper("spec.volumes", VolumeMixin._build_volumes)]
+    ] = None
+    volumes: Annotated[VolumesT, _WorkflowModelMapper("spec.volumes", VolumeMixin._build_volumes)] = None
 
     # Hera-specific fields
     workflows_service: Optional[WorkflowsService] = None