sessionphase: fix service latency computation in an error case

[yuzefovich]

If we encounter an error during the logical planning, then the end
execution phase is never set. Previously, we would use that unset value
in order to compute the latency of planning and execution and would get
a negative result. This is now fixed.

Release note: None

[cockroach-teamcity]

This change is 

[maryliag]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @Azhng)

[yuzefovich]

TFTR!

bors r+

[craig]

This PR was included in a batch that was canceled, it will be automatically retried

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 2257e81 to blathers/backport-release-21.1-72493: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.1.x failed. See errors above.

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