Fix : Next hydration error - empty Fragment

- 빈 fragment 문법 (<> </>)을 사용했을 때 나는 오류 제거
- fragment 문법을 사용하려면,
```js
import { Fragment } from "react";
...

return (
  <Fragment>
  </Fragment>
)
와 같이 명시적으로 작성해 주어야 하는 것 같다.

Relate to:
[Frontend-board : /board/index.js (게시판 메인 페이지) #18
Make components for Frontend-board  #17

frontend/components/Layout.js

@@ -8,11 +8,11 @@ export default function Layout({ children }) {
     const routerObj = { '/': 'Home', '/about': 'About' }
     //console.log(router.pathname)
     return (
-        <>
+        <div>
             {/*<Seo title={routerObj[router.pathname]} /> */}
             <Seo title={router.pathname} />
             <NavBar />
             <div>{children}</div>
-        </>
+        </div>
     )
 }

frontend/components/board/BoardCenter.js

@@ -2,7 +2,7 @@
 
 export default function BoardCenter() {
     return (
-        <>
-        </>
+        <div>
+        </div>
     )
 }

frontend/components/board/BoardMiniView.js

@@ -2,7 +2,7 @@
 
 export default function BoardMiniView(props) {
   return (
-    <>
+    <div>
       {/* type:string 에 따라 백에서 다른 값 받아옴. */}
 
       <table>
@@ -55,7 +55,7 @@ export default function BoardMiniView(props) {
 
       <style jsx>{`
         table {
-          margin: 0px;
+          margin: 10px;
           border: 1px solid purple;
           width: 300px;
         }
@@ -72,6 +72,6 @@ export default function BoardMiniView(props) {
           flex-direction: row;
         }
       `}</style>
-    </>
+    </div>
   )
 }

frontend/components/board/BoardNavBar.js

@@ -2,7 +2,7 @@
 
 export default function BoardNavBar() {
   return (
-    <>
+    <div>
       <ul>
         {/*게시물 좌측*/}
         <li>
@@ -67,6 +67,6 @@ export default function BoardNavBar() {
         </li>
 
       </ul>
-    </>
+    </div>
   )
 }

frontend/components/board/BoardSearchBar.js

@@ -2,14 +2,14 @@
 
 export default function BoardSearchBar() {
   return (
-    <>
+    <div>
       {/* bootstrap */}
       <nav class="navbar navbar-light bg-light">
         <form class="form-inline">
           <input class="form-control mr-sm-2" type="search" placeholder="게시판 검색" aria-label="Search" />
           <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
         </form>
       </nav>
-    </>
+    </div>
   )
 }

frontend/components/board/BoardUser.js

@@ -2,7 +2,7 @@
 
 export default function BoardUser(props) {
   return (
-    <>
+    <div>
       <div className={"box"}>
         <div className={"image"}>이미지</div>
         <div className={"userId"}>유저아이디</div>
@@ -26,6 +26,6 @@ export default function BoardUser(props) {
           
         }
       `}</style>
-    </>
+    </div>
   )
 }