Fixing some issues with null text box values

ui/src/components/pre-game.component/join-game-input.component.jsx

@@ -8,7 +8,7 @@ export default ({ gameId, onChangeGameId, onJoinGame, cannotJoinGame }) => (
     alignItems: "center",
   }}>
     <span style={{ padding: "0.1em" }}>Game ID:</span>
-    <input style={{ margin: "0.1em" }} type="text" value={gameId} onChange={onChangeGameId} />
+    <input style={{ margin: "0.1em" }} type="text" value={gameId || ""} onChange={onChangeGameId} />
     <button style={{ margin: "0.1em" }} onClick={onJoinGame} disabled={cannotJoinGame}>
       Join Existing Game
     </button>

ui/src/components/pre-game.component/player-name-input.component.jsx

@@ -8,6 +8,6 @@ export default ({ playerName, onChangePlayerName }) => (
     alignItems: "center",
   }}>
     <span style={{ padding: "0.2em" }}>Player Name</span>
-    <input style={{ padding: "0.1em" }} type="text" value={playerName} onChange={onChangePlayerName} />
+    <input style={{ padding: "0.1em" }} type="text" value={playerName || ""} onChange={onChangePlayerName} />
   </div>
 );