Fix JiveXML loader crash when optional XML tags are missing

[remo-lab]

Description

This PR fixes a crash in the JiveXML loader caused by unguarded access to missing XML tags.

Several parsing methods in jivexml-loader.ts assume that certain XML tags are always present and directly access getElementsByTagName(...)[0].innerHTML. When a tag is missing (which happens in real JiveXML files from older versions or different experiments), this results in a runtime TypeError and prevents the entire event from loading.

Instead of failing hard on a single missing field, the parser should degrade gracefully and continue loading available data.

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What was happening

• getElementsByTagName() returns an empty collection when a tag is missing
• Accessing [0].innerHTML on that result throws at runtime
• One missing optional tag causes the entire JiveXML file to fail to load
• Users see an empty event display with no clear explanation

This pattern appears in multiple parsing functions (e.g. vertices, clusters, tracks).

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What this change does

• Adds defensive checks when reading XML tag contents
• Treats missing tags as optional instead of fatal
• Prevents runtime crashes due to malformed or incomplete XML
• Allows partially valid JiveXML files to load successfully

The change is intentionally minimal and does not alter behavior when tags are present.

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How to reproduce (before this fix)

1. Load a JiveXML file containing a <RVx> collection without optional fields such as <chi2>
2. The loader throws Cannot read properties of undefined (reading 'innerHTML')
3. No event data is rendered

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Impact

• Improves robustness of the JiveXML loader
• Restores compatibility with older or variant JiveXML formats
• Prevents total failure due to a single missing field
• Improves user experience by avoiding silent blank displays

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Scope and risk

• No API changes
• No schema changes
• Low risk: defensive checks only
• Does not affect valid XML files

[remo-lab]

Hi @EdwardMoyse 👋,
This addresses a crash in the JiveXML loader when optional XML tags are missing by adding simple defensive checks.

Whenever you have time, I’d appreciate a review. Thanks!