[Kafka API] fix auth flags check

[a-serebryanskiy]

Changelog entry

Issue: #18598

Original PR to main: #18594

Fixes bug with not forwarding auth token in Kafka proxy in absence of old flag.

Changelog category

• Bugfix

Description for reviewers

...

[github-actions]

⚪ 2025-05-21 08:29:23 UTC Pre-commit check for f9e7e07 has started.
⚪ 2025-05-21 08:32:04 UTC Build linux-x86_64-release-asan is running...
🟢 2025-05-21 08:55:12 UTC Build successful.
⚪ 2025-05-21 08:55:29 UTC Tests are running...
🔴 2025-05-21 11:04:54 UTC Some tests failed, follow the links below.

Test history | Test log

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
10198	10093	0	33	30	42

🟢 2025-05-21 11:05:42 UTC ydbd size 5.6 GiB changed* by +1.4 KiB, which is < 100.0 KiB vs stable-24-4: OK

ydbd size dash	stable-24-4: 0956375	merge: f9e7e07	diff	diff %
ydbd size	6 028 267 152 Bytes	6 028 268 592 Bytes	+1.4 KiB	+0.000%
ydbd stripped size	1 508 554 496 Bytes	1 508 554 944 Bytes	+448 Bytes	+0.000%

^{*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]

⚪ 2025-05-21 08:29:25 UTC Pre-commit check for f9e7e07 has started.
⚪ 2025-05-21 08:32:05 UTC Build linux-x86_64-relwithdebinfo is running...
🟢 2025-05-21 09:09:54 UTC Build successful.
⚪ 2025-05-21 09:10:10 UTC Tests are running...
🔴 2025-05-21 10:37:47 UTC Some tests failed, follow the links below.

Test history | Test log

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
14447	13087	0	19	1301	40

🟢 2025-05-21 10:38:43 UTC ydbd size 8.3 GiB changed* by +1.0 KiB, which is < 100.0 KiB vs stable-24-4: OK

ydbd size dash	stable-24-4: 0956375	merge: f9e7e07	diff	diff %
ydbd size	8 898 883 424 Bytes	8 898 884 448 Bytes	+1.0 KiB	+0.000%
ydbd stripped size	485 045 768 Bytes	485 045 960 Bytes	+192 Bytes	+0.000%

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