Prevent HashOperations MultiGet from returning null values in the collection

[ShaneLee]

This PR addresses issue #2309. It prevents the multi-get operation from returning null values inside the collection. This is an issue that can be very difficult to diagnose as it leads to stack traces without reference to the client code.

I've raised this PR against main for now. I expect it would be better going into 2.7.x, but happy to put it into another branch.

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