Partial merge and empty sink level

[martinsumner]

The addition of partial merge doesn't handle a merge that creates and empty result.

For example, if a set of files containing nothing but leveled tombstones are handled in the partial merge:

leveled/src/leveled_pclerk.erl

Line 394 in 69e8b29

{KL2Additions, [], []} = add_entry(NewMergeKL2, FNSnk, TS2, Additions),

This might return {empty, [], Additions} not {KL2Additions, [], []}. This will cause the clerk to crash, and this should ripple up to the store crashing.

riak_SUITE:test_large_lsm_merge/1 may detect this rarely.