community: Fix TypeError in PebbloRetrievalQA (#23170)
**Description:**
Fix "`TypeError: 'NoneType' object is not iterable`" when the
auth_context is absent in PebbloRetrievalQA. The auth_context is
optional; hence, PebbloRetrievalQA should work without it, but it throws
an error at the moment. This PR fixes that issue.
**Issue:** NA
**Dependencies:** None
**Unit tests:** NA
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Co-authored-by: Chester Curme <chester.curme@gmail.com>
