Fix incorrect nil handling in workflowTaskPoller

[shijiesheng]

What changed?

• explicit nil handling in newWorkflowTaskPoller for ldaTunnel

Why?

Golang doesn't handle passing in nil pointer of concrete type directly into the interface. It still considers it not nil, which will cause panic in this case.

How did you test it?

Unit test fail before and pass after the change

Potential risks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.59%. Comparing base (9ffbb1f) to head (ffe6726).
Report is 1 commits behind head on master.

Files with missing lines	Coverage Δ
internal/internal_task_pollers.go	82.76% <100.00%> (+0.12%)	⬆️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9ffbb1f...ffe6726. Read the comment docs.