ddtrace/tracer: Fix panic on Extract when DD_TRACE_PROPAGATION_EXTRACT_FIRST=true and no trace headers present

[mtoffl01]

What does this PR do?

Fixes a panic caused by this line of code: return extractedCtx.(*spanContext), nil, when extractedCtx is nil.

Motivation

Bug was detected when configuring DD_TRACE_PROPAGATION_EXTRACT_FIRST=true and no tracing-related keys are found in carrier. To reproduce this, you can paste the TestExtractNoHeaders to the v1.71.0 branch and watch it panic.
Bug was introduced in v1.71.0 as a result of this PR.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

Unsure? Have a question? Request a review!

[datadog-datadog-prod-us1]

Datadog Report

Branch report: mtoff/extract-edgecase
Commit report: ca210a1
Test service: dd-trace-go

✅ 0 Failed, 5220 Passed, 73 Skipped, 1m 22.33s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-31 18:42:34

Comparing candidate commit d0a04eb in PR branch mtoff/extract-edgecase with baseline commit 7dec46c in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 59 metrics, 0 unstable metrics.

[kakkoyun]

This looks good to me, as far I can tell.

There is a smoke test failing. Is it related?