[SVLS-3853] handle empty function case in tag-object

[joeyzhao2018]

What does this PR do?

Fixes #464 (again, 😭 )

Motivation

Testing Guidelines

Additional Notes

Types of Changes

• [x] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [ ] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog
• [ ] This PR passes the integration tests (ask a Datadog member to run the tests)

[codecov-commenter]

Codecov Report

Attention: 117 lines in your changes are missing coverage. Please review.

Comparison is base (35295b3) 80.35% compared to head (d57c29f) 82.12%. Report is 99 commits behind head on main.

:exclamation: Current head d57c29f differs from pull request most recent head 626f525. Consider uploading reports for the commit 626f525 to get more accurate results

Files	Patch %	Lines
src/trace/listener.ts	53.84%	17 Missing and 1 partial :warning:
src/trace/span-context-wrapper.ts	57.57%	13 Missing and 1 partial :warning:
src/trace/span-inferrer.ts	83.33%	0 Missing and 10 partials :warning:
src/trace/xray-service.ts	91.50%	8 Missing and 1 partial :warning:
src/trace/step-function-service.ts	92.13%	6 Missing and 1 partial :warning:
src/trace/cold-start-tracer.ts	87.23%	2 Missing and 4 partials :warning:
src/utils/event-type-guards.ts	14.28%	6 Missing :warning:
src/trace/context/extractor.ts	91.07%	5 Missing :warning:
src/trace/context/extractors/sqs.ts	66.66%	4 Missing and 1 partial :warning:
src/index.ts	91.66%	3 Missing and 1 partial :warning:
... and 13 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #467 +/- ## ========================================== + Coverage 80.35% 82.12% +1.77% ========================================== Files 36 54 +18 Lines 1761 2188 +427 Branches 410 510 +100 ========================================== + Hits 1415 1797 +382 - Misses 294 329 +35 - Partials 52 62 +10 ```

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