Fix p_mask cls token masking in question-answering pipeline
# What does this PR do?
It fixes a really small bug described in detail in the issue - it only adds a condition to the if statement responsible for unmasking the `cls_token_id` in the `p_mask` used in the question answering pipeline.

Fixes #10810 

## Who can review?
Anyone in the community is free to review the PR once the tests have passed. 