fix(ui): guard against null/undefined dates in Gantt chart to prevent RangeError

[idrisakorede]

Fix Grid View crash (RangeError: Invalid time value) when viewing tasks in non-terminal states (scheduled, running) with "Show Gantt" enabled. This is a regression from 3.1.7 introduced by #61250.

Root cause: dayjs(value).toISOString() is called on null/undefined date values for tasks that haven't completed yet.

Changes in utils.ts:

• Group/mapped task guard: === null → falsy check (!value) to also catch undefined
• Individual task tries: added !tryInstance.end_date fallback so null end dates use current time instead of crashing

No test coverage or behavior changes beyond the null guard. Tasks with missing end dates now render a bar extending to "now" (consistent with running task behavior).

closes: #63954

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[potiuk]

@idrisakorede This PR has been converted to draft because it does not yet meet our Pull Request quality criteria.

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