Adding a null check before initiating the internal Activity

[RohitRanjanMS]

Adding a null check before starting the internal Activity in the InvokeFunctionAsync method. This will help in scenarios where customers want to consume the request telemetry emitted by AspNetCoreInstrumentation. Adding this null check ensures that if a request telemetry already exists (generated by AspNetCoreInstrumentation), the internal Activity won’t be started.

resolves #2733

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

[jviau]

LGTM.

Something I realize we are missing with our approach of new'ing up the activity manually: we are missing out on a perf optimization to not do this at all if the user is not listening to activities. How we will do this I am unsure, but it is a behavior with the ActivitySource API: if no one is listening to the source, it no-ops on activity creation.