Fold pinned existence check into __wt_txn_pinned_timestamp

There are three callers of __wt_txn_pinned_timestamp, two of them use a check for

 if (S2C(session)->txn_global.has_pinned_timestamp)

before making the function call and the other doesn't.

We should see if we can safely fold the check into __wt_txn_pinned_timestamp, or if not try to help callers do the right thing if there is no pinned timestamp.

There is possibly a correctness problem with the code that isn't checking for the existence of a pinned timestamp before making the call, so I think we should prioritize this work.