fix: prevent infinite loop on TraceDiff with empty trace IDs

[Parship12]

Which problem is this PR solving?

• Navigating to /trace/... (the compare URL with no trace IDs filled in) showed a blank page.

Root cause

Two values that both mean "no trace selected" were compared with strict
equality in syncStates:

• The Zustand store initializes a and b as null
• URL parsing produces undefined when a trace ID segment is empty undefined !== null is true, so syncStates called forceState on every render. forceState wrote null back to the store, which triggered a re-render, which re-ran the effect, which called forceState again - infinitely.

In Console:

Before:

After:

How was this change tested?

• Unit testing
• Compare page working properly

Checklist

• I have read https://github.com/jaegertracing/jaeger/blob/main/CONTRIBUTING_GUIDELINES.md
• I have signed all commits
• I have added unit tests for the new functionality
• I have run lint and test steps successfully: make lint test

AI Usage in this PR (choose one)

See AI Usage Policy.

• None: No AI tools were used in creating this PR
• Light: AI provided minor assistance (formatting, simple suggestions)
• Moderate: AI helped with code generation or debugging specific parts
• Heavy: AI generated most or all of the code changes

[Copilot]

Pull request overview

This PR fixes an infinite loop issue that occurred when navigating to the trace comparison URL with no trace IDs (e.g., /trace/...). The root cause was a mismatch between how the Zustand store (which uses null for empty trace IDs) and the URL parser (which produces undefined for empty segments) represented the same concept, causing syncStates to continuously trigger state synchronization. The fix normalizes both undefined and null to null in the comparison using the nullish coalescing operator, preventing the infinite loop.

Changes:

• Modified the syncStates function to normalize undefined and null values when comparing URL and store state for trace IDs

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.21%. Comparing base (f9226f8) to head (f534e73).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3714   +/-   ##
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  Coverage   89.21%   89.21%           
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  Files         330      330           
  Lines        9900     9900           
  Branches     2570     2572    +2     
=======================================
  Hits         8832     8832           
  Misses        926      926           
  Partials      142      142           

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