Add missing Optional on a few nullable fields (#4076)

## What type of PR is this? (check all applicable)

- [x] Refactor

## Have you discussed this change with the InvokeAI team?
- [ ] Yes
- [x] No, because: trivial

## Description

Adds a few obviously missing `Optional` on fields that default to
`None`.

invokeai/app/api/dependencies.py

@@ -1,5 +1,6 @@
 # Copyright (c) 2022 Kyle Schouviller (https://github.com/kyle0654)
 
+from typing import Optional
 from logging import Logger
 import os
 from invokeai.app.services.board_image_record_storage import (
@@ -54,7 +55,7 @@ logger = InvokeAILogger.getLogger()
 class ApiDependencies:
     """Contains and initializes all dependencies for the API"""
 
-    invoker: Invoker = None
+    invoker: Optional[Invoker] = None
 
     @staticmethod
     def initialize(config: InvokeAIAppConfig, event_handler_id: int, logger: Logger = logger):

invokeai/backend/install/model_install_backend.py

@@ -7,7 +7,7 @@ import warnings
 from dataclasses import dataclass, field
 from pathlib import Path
 from tempfile import TemporaryDirectory
-from typing import List, Dict, Callable, Union, Set, Optional
+from typing import Optional, List, Dict, Callable, Union, Set
 
 import requests
 from diffusers import DiffusionPipeline
@@ -86,8 +86,8 @@ class ModelLoadInfo:
     name: str
     model_type: ModelType
     base_type: BaseModelType
-    path: Path = None
+    path: Optional[Path] = None
-    repo_id: str = None
+    repo_id: Optional[str] = None
     description: str = ""
     installed: bool = False
     recommended: bool = False

invokeai/backend/model_management/model_manager.py

@@ -276,7 +276,7 @@ class ModelInfo:
     hash: str
     location: Union[Path, str]
     precision: torch.dtype
-    _cache: ModelCache = None
+    _cache: Optional[ModelCache] = None
 
     def __enter__(self):
         return self.context.__enter__()