Fix Leaderboard nulls

[McLaynV]

Fix Leaderboard null errors in the Leaderboard.

These errors occur when running build:client:

> vue-ts-template@1.0.0 build   
> vue-tsc --noEmit && vite build

src/components/Leaderboard.vue:45:30 - error TS2345: Argument of type 'number | undefined' is not assignable to parameter of type 'number'.
  Type 'undefined' is not assignable to type 'number'.

45         {{ msToElapsedString(store.submittedRun?.totalTimeMilliseconds) }}
                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/components/Leaderboard.vue:62:21 - error TS2531: Object is possibly 'null'.

62             :class="store.submittedRun.id === result.id ? 'is-selected' : ''"
                       ~~~~~~~~~~~~~~~~~~

src/components/Leaderboard.vue:85:21 - error TS2531: Object is possibly 'null'.

85             :class="store.submittedRun.id === result.id ? 'is-selected' : ''"
                       ~~~~~~~~~~~~~~~~~~


Found 3 errors in the same file, starting at: src/components/Leaderboard.vue:45