fix(frontend): reject null payloads in dashboard type predicates

[eugenegujing]

What changes were proposed in this PR?

The five type guards in dashboard/type/type-predicates.ts checked their nested payload with typeof value.<field> === "object", which also accepts null because typeof null === "object" in JavaScript, so an entry like {workflow: null} passed isDashboardWorkflow and the DashboardEntry constructor then crashed with an unrelated TypeError: Cannot read properties of null while dereferencing the payload, instead of reaching its intentional "Unexpected type in DashboardEntry." error path. The guards also returned the falsy input itself rather than false for null/undefined input, violating their declared boolean type-guard signatures.

This PR adds an isNonNullObject type guard (typeof x === "object" && x !== null) to the shared predicate utility (common/util/predicate.ts, next to isDefined) so the correct non-null object check is discoverable and reusable, uses it in the four object-payload guards, and switches all five guards to !!value && ... so they return strict booleans in every path. isDashboardProject's !value.workflow exclusion is deliberately unchanged: a null workflow field means "no workflow data", so an object with a string name still classifies as a project and no valid entry changes classification.

Any related issues, documentation, discussions?

Fixes #6439. The buggy behavior was pinned by the five "(current behavior)" tests added in #6425 (issue #6400), which this PR flips to assert the corrected behavior.

How was this PR tested?

Added predicate.spec.ts (8 tests) covering isNonNullObject and the previously untested isDefined. Updated type-predicates.spec.ts: the four null-payload pins now assert toBe(false), the null/undefined input cases are tightened from toBeFalsy() to toBe(false), and the isDashboardProject null-workflow case keeps asserting true with a comment marking it as an intentional decision; the 5x5 cross-classification matrix and all of its 25 expected values are untouched, confirming that classification of realistic entries is unaffected. Ran locally via yarn ng test --watch=false --include='**/common/util/predicate.spec.ts' --include='**/dashboard/type/type-predicates.spec.ts' (61/61 pass), plus the full frontend suite whose failure set is identical to the unmodified main baseline (pre-existing, environment-related failures only), and tsc --noEmit with zero errors.

Was this PR authored or co-authored using generative AI tooling?

Co-authored using Claude Code(Fable 5).

[github-actions]

Automated Reviewer Suggestions

Based on the git blame history of the changed files, we recommend the following reviewers:

• Contributors with relevant context: @aglinxinyuan, @gracecluvohio
  You can notify them by mentioning @aglinxinyuan, @gracecluvohio in a comment.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.17%. Comparing base (aa0bc44) to head (bbbb7b3).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6443      +/-   ##
============================================
- Coverage     70.17%   70.17%   -0.01%     
  Complexity     3388     3388              
============================================
  Files          1142     1142              
  Lines         44844    44846       +2     
  Branches       4949     4950       +1     
============================================
+ Hits          31471    31472       +1     
  Misses        11739    11739              
- Partials       1634     1635       +1     

Flag	Coverage Δ		*Carryforward flag
access-control-service	70.00% <ø> (ø)		Carriedforward from aa0bc44
agent-service	76.76% <ø> (ø)		Carriedforward from aa0bc44
amber	66.67% <ø> (ø)		Carriedforward from aa0bc44
computing-unit-managing-service	9.97% <ø> (ø)		Carriedforward from aa0bc44
config-service	52.30% <ø> (ø)		Carriedforward from aa0bc44
file-service	65.63% <ø> (ø)		Carriedforward from aa0bc44
frontend	68.91% <100.00%> (+<0.01%)	⬆️
notebook-migration-service	78.94% <ø> (ø)		Carriedforward from aa0bc44
pyamber	91.19% <ø> (ø)		Carriedforward from aa0bc44
workflow-compiling-service	55.14% <ø> (ø)		Carriedforward from aa0bc44

*This pull request uses carry forward flags. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.