Avoid overwriting default jaeger values with nil

What does this MR do?

Avoid overwriting default jaeger values with nil

During the review process for adding opentracing factories, a bug was introduced which caused Jaeger to initialize an invalid tracer. The bug was due to use sending nil through as a kwarg when the Jaeger initializer used a non-nil default value.

This bug is fairly insidious as, the tracer looks like a tracer, but, when methods are invoked, it throws NoMethodError errors. To ensure that this issue does not happen in future, the tests have been changed to ensure that the tracer works as expected. This could avoid problems in future when upgrading to newer versions of

What are the relevant issue numbers?

Fix for bug introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24186

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer