[Nexthop][wedge800bnhp] Avoid creating TH5 port serdes with zeroed TX FIR / empty Preemphasis

[andrew-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

On Tomahawk5, recreating a port serdes under zeroPreemphasis=true (set by LinkStateToggler in HW loopback tests) crashes during a VCO change that forces a port recreate:

• For optical profiles, avoid creating the serdes object with zeroed TxFirMain values. Create it without the TX FIR taps, then program the zeroed taps in a follow-up update accepted by the SDK.
• Preserve the existing copper-profile path and avoid programming an empty Preemphasis attribute when no TX lanes are present.

Test Plan

wedge800bnhp:

Running all tests took 0:00:46.291503 between 2026-06-15 22:35:01.767058 and 2026-06-15 22:35:48.058561
[ PASSED ] cold_boot.AgentPortBandwidthTest/AgentPortBandwidthParamTest.VerifyPortRateTraffic/1 (10631 ms)
[ PASSED ] warm_boot.AgentPortBandwidthTest/AgentPortBandwidthParamTest.VerifyPortRateTraffic/1 (9513 ms)
[ PASSED ] cold_boot.AgentPortBandwidthTest/AgentPortBandwidthParamTest.VerifyPortRateTraffic/3 (10583 ms)
[ PASSED ] warm_boot.AgentPortBandwidthTest/AgentPortBandwidthParamTest.VerifyPortRateTraffic/3 (9488 ms)
Summary:
   PASSED : 4
   FAILED : 0
   SKIPPED : 0
   TIMEOUT : 0

[meta-codesync]

@tanmayaudupa has imported this pull request. If you are a Meta employee, you can view this in D121230677.

[meta-codesync]

@tanmayaudupa merged this pull request in 01d4611.