Servlet: avoid NPE when headerNames is null

[amarziali]

What Does This Do

headerNames can return null as per the servlet javadoc. This PR puts some guards to avoid errors like:

java.lang.NullPointerException
  at datadog.trace.instrumentation.servlet3.HttpServletExtractAdapter.forEachKey(HttpServletExtractAdapter.java:20)
  at datadog.trace.bootstrap.instrumentation.api.AgentPropagation$ContextVisitor.forEachKeyValue(AgentPropagation.java:49)
  at

Motivation

Additional Notes

Contributor Checklist

• Format the title according to the contribution guidelines
• Assign the type: and (comp: or inst:) labels in addition to any other useful labels
• Avoid using close, fix, or any linking keywords when referencing an issue
  Use solves instead, and assign the PR milestone to the issue
• Update the CODEOWNERS file on source file addition, migration, or deletion
• Update public documentation with any new configuration flags or behaviors

Jira ticket: [PROJ-IDENT]

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[ValentinZakharov]

LGTM

[dd-octo-sts]

/merge

[gh-worker-devflow-routing-ef8351]

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2026-05-18 09:56:07 UTC ℹ️ Start processing command /merge

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2026-05-18 09:56:12 UTC ℹ️ MergeQueue: pull request added to the queue

The expected merge time in master is approximately 1h (p90).

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2026-05-18 11:05:09 UTC ℹ️ MergeQueue: This merge request was merged