[dashboard] Guard memoryConverter against null/undefined/NaN from psutil stats

[daiping8]

Closes #64401

Motivation

The dashboard's Worker info view is unrenderable on nodes where the reporter agent emits null for any memoryInfo field (e.g. pfaults/pageins when psutil cannot read them or raises AccessDenied). memoryConverter(null)
crashes with Cannot read properties of null (reading 'toFixed'), taking the whole Worker table down. This makes the per-node Worker view unusable for users on affected platforms/configurations.

Implementation Details

Two files changed under python/ray/dashboard/client/src/util/:

1. converter.ts

   • Widened the parameter type to number | undefined | null to reflect that psutil as_dict() can emit null.
   • Added an upfront guard: if bytes is null, undefined, not a number, or NaN, return "-" (an explicit no-data placeholder) instead of falling through to .toFixed. The "-" placeholder is clearer in a table than an empty cell.
   • Existing byte-formatting branches and zero/negative behavior are left unchanged (zero still renders as 0.0000B); this change only adds the invalid-value guard, keeping the fix focused on the root cause.

2. converter.unit.test.ts

   • Added an edge cases describe block with regression tests asserting that memoryConverter(null), memoryConverter(undefined), and memoryConverter(NaN)each do not throw and return"-"`.

The reporter-agent / serialization path is intentionally untouched: psutil returning null for unreadable fields is expected behavior, so the correct fix is to tolerate it at the formatting boundary rather than to coerce it earlier.

Verification

• Unit tests pass:

  cd python/ray/dashboard/client
  CI=true ./node_modules/.bin/react-scripts test --watchAll=false src/util/converter.unit.test.ts
  # Tests: 9 passed, 9 total

• Manual verification on a running cluster:

  • npm run build to rebuild the dashboard frontend.
  • ray start --head --dashboard-host=127.0.0.1 --dashboard-port=8265.
  • Open http://127.0.0.1:8265/#/cluster/nodes/<node-id>, open the Worker section — the page renders and no toFixed TypeError appears in the console, even when worker memoryInfo fields are null.

[gemini-code-assist]

Code Review

This pull request updates the memoryConverter utility to safely handle null, undefined, and NaN values by returning "-" instead of crashing, and adds corresponding unit tests to verify this behavior. The review feedback suggests simplifying the guard condition in memoryConverter by removing redundant checks and using Number.isNaN instead of the global isNaN. Additionally, it recommends cleaning up the unit tests by removing redundant not.toThrow assertions.