Fix ydb workload vector sampling with nullable PK

[vitalif]

Changelog category

• Not for changelog (changelog entry is not required)

Description for reviewers

Fixes ydb workload vector run select auto-sampling if the table's primary key is nullable.

[github-actions]

🟢 2025-08-01 16:35:01 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-08-01 17:10:32 UTC Pre-commit check linux-x86_64-release-asan for c36305e has started.
⚪ 2025-08-01 17:10:47 UTC Artifacts will be uploaded here
⚪ 2025-08-01 17:13:27 UTC ya make is running...
🟡 2025-08-01 18:21:13 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
1439	1211	0	26	196	6

🟢 2025-08-01 18:21:25 UTC Build successful.
🟡 2025-08-01 18:21:42 UTC ydbd size 3.9 GiB changed* by +413.4 KiB, which is >= 100.0 KiB vs main: Warning

ydbd size dash	main: 6560122	merge: c36305e	diff	diff %
ydbd size	4 236 558 400 Bytes	4 236 981 752 Bytes	+413.4 KiB	+0.010%
ydbd stripped size	1 467 010 776 Bytes	1 467 135 608 Bytes	+121.9 KiB	+0.009%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[github-actions]

⚪ 2025-08-01 17:10:34 UTC Pre-commit check linux-x86_64-relwithdebinfo for c36305e has started.
⚪ 2025-08-01 17:10:49 UTC Artifacts will be uploaded here
⚪ 2025-08-01 17:13:27 UTC ya make is running...
🟢 2025-08-01 17:58:42 UTC Tests successful.

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
2233	2226	0	0	6	1

🟢 2025-08-01 17:59:00 UTC Build successful.
🟡 2025-08-01 17:59:11 UTC ydbd size 2.2 GiB changed* by +347.9 KiB, which is >= 100.0 KiB vs main: Warning

ydbd size dash	main: 6560122	merge: c36305e	diff	diff %
ydbd size	2 409 631 632 Bytes	2 409 987 880 Bytes	+347.9 KiB	+0.015%
ydbd stripped size	503 302 344 Bytes	503 403 624 Bytes	+98.9 KiB	+0.020%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}