Avoid null collection on decoded spans

[PerfectSlayer]

What Does This Do

This PR updates the decoded spans to avoid null collections.

Motivation

This changes will help asserting the trace / span structure and values with the upcoming rules and matchers.

Additional Notes

Contributor Checklist

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Jira ticket: APMLP-1247

[chatgpt-codex-connector]

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[datadog-datadog-prod-us1]

More details

The decoder changes consistently replace absent meta_struct values with immutable empty maps across v0.4, v0.5, and v1, matching the updated smoke-test assertions. No diff-only behavioral regression was identified; local execution was attempted but the sandbox lacks the required Gradle distribution and Java 25 toolchain.

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[datadog-datadog-prod-us1]

🎯 Code Coverage (details)
• Patch Coverage: 0.00%
• Overall Coverage: 57.54% (+0.02%)

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[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	14.88 s	14.68 s	[+0.6%; +2.1%] (maybe worse)
startup:insecure-bank:tracing:Agent	13.64 s	13.75 s	[-1.7%; +0.1%] (no difference)
startup:petclinic:appsec:Agent	17.01 s	16.85 s	[-0.1%; +2.1%] (no difference)
startup:petclinic:iast:Agent	16.90 s	16.97 s	[-1.2%; +0.4%] (no difference)
startup:petclinic:profiling:Agent	16.52 s	16.82 s	[-3.2%; -0.4%] (maybe better)
startup:petclinic:sca:Agent	16.97 s	16.80 s	[+0.1%; +1.9%] (maybe worse)
startup:petclinic:tracing:Agent	16.13 s	16.16 s	[-1.0%; +0.5%] (no difference)

Commit: d1a28728 · CI Pipeline · Benchmarking Platform UI

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Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.

[jandro996]

LGTM! Thanks!