add constraint if no team, button not display

src/view/nav-bar.js

@@ -121,7 +121,7 @@ export default{
 					:nom="team.name"
 					:description="team.description">
 	 			</team-card>
-				<input type="submit" value="Export" v-on:click="Export"/>
+				<input type="submit" value="Export" v-if="teams.length>0" v-on:click="Export" />
 			</div>
 		`
 }