Don't show "empty" dates

ui/src/song/SongList.js

@@ -1,6 +1,5 @@
 import React from 'react'
 import {
-  DateField,
   Filter,
   FunctionField,
   NumberField,
@@ -10,6 +9,7 @@ import {
 import { useMediaQuery } from '@material-ui/core'
 import FavoriteIcon from '@material-ui/icons/Favorite'
 import {
+  DateField,
   DurationField,
   List,
   SongContextMenu,