ui: do not show current time as a fall back for empty timestamps on Jobs pages

[koorosh]

Previously, timestamps for running jobs showed
current time even if job is not finished because
TimestampToMoment function used current time as
a default value for Nulls.
With this change it is explicitly set to Null to
make sure that Job's time is not misinterpreted.

Resolves: #109204

Release note (ui change): Correctly display timestamps
for creation, last modified, and completed time on Jobs table.

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[cockroach-teamcity]

This change is 

[maryliag]

why on your image the alter tables don't have a completed time? Shouldn't they have it? The issue was focusing on jobs being executed, so I would also like to see examples of that

Reviewed 2 of 3 files at r1, all commit messages.
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[koorosh]

I made backend to return job with empty finished_at field just to simplify testing as it is pure front-end issue so that's why completed job doesn't have "Completed time".
In addition I'd like to highlight that the problem is not specifically related for running jobs. The problem in general is that frontend changed timestamp's values from 'NULL' to current time.
With current change Jobs table display the same data as SHOW JOBS query.

Appreciate any help on how to have a long running job for testing under real conditions 🙏🏽

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @gtr, @j82w, @THardy98, @xinhaoz, and @zachlite)

[maryliag]

@yuzefovich would you mind giving some repro steps from the issue you created? How to get a long running job so Andrii can test his changes?

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @gtr, @j82w, @THardy98, @xinhaoz, and @zachlite)

[yuzefovich]

I don't have reproduction steps - I noticed this while looking at DB Console of one of existing CC clusters (that I was investigating for a different reason). Some context can be found in https://cockroachlabs.slack.com/archives/C04N0AS14CT/p1692666351682189. Perhaps @fqazi can help with a repro.

[maryliag]

Reviewed 1 of 3 files at r1.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @gtr, @j82w, @THardy98, @xinhaoz, and @zachlite)

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[koorosh]

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

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