Handle empty data nodes when finding last key

[geoffxy]

This PR addresses an edge case in get_payload_last_no_greater_than() and find_last_no_greater_than() related to querying a key larger than the largest key in the index.

The get_payload_last_no_greater_than() method should return the record associated with the largest key in ALEX. But the method actually returns an invalid value. What appears to be happening is that the get_leaf() call returns an empty leaf. That leaf’s previous leaf is also empty. You actually have to traverse backwards through a few empty leaf nodes until you reach the last populated node, which stores the record with the largest key.

As discussed offline, an acceptable solution is to just traverse backwards to the first non-empty data node (if it exists).

I've also included a test case that reproduces the problem and currently fails on master.

[ghost]

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