internal/civisibility: add more nil checks to increase resilience

[tonyredondo]

What does this PR do?

Add more nil checks on the CI Visibility library to either workaround possible crash, or panic with a better error message.

Motivation

Try to increase library resilience

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.

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