Why are these changes needed?

There are several fields that are actually nullable from the gpustat library that the dashboard front-end had as non-nullable types. This led to the page crashing in scenarios where gpu utilization or memory utilization was not available.

Related issue number

Closes #11313

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

Manually QA'd but not tested, like much of the front-end...