refactor: Make the pic optional in create forum post function

src/service/firestore/forum/createPost.ts

@@ -6,15 +6,16 @@ type createPostParam = {
   userId: string;
   title: string;
   content: string;
+  pic?: string;
 };
 
 export const createPost = async (post: createPostParam) => {
   const db = firebase.firestore();
   await db.collection("threads_" + getEnv()).add({
     ...post,
+    pic: post.pic || "",
     commentCount: 0,
     likeCount: 0,
-    pic: "",
     timestamp: firebase.firestore.FieldValue.serverTimestamp(),
   });
 };