fix(microcmsClient): 環境変数が未定義の場合のデフォルト値を設定

src/lib/microcmsClient.ts

@@ -1,6 +1,6 @@
 import { createClient } from "microcms-js-sdk";
 
 export const microcmsClient = createClient({
-  serviceDomain: process.env.MICROCMS_SERVICEDOMAIN as string,
-  apiKey: process.env.MICROCMS_API_KEY as string,
+  serviceDomain: process.env.MICROCMS_SERVICEDOMAIN || "",
+  apiKey: process.env.MICROCMS_API_KEY || "",
 });