Backend failure for witness with tree size 0

Running against a newly added witness at tree size 0:

May 17 12:23:35 poc.sigsum.org sigsum-log-start.sh[179959]: 2023/05/17 12:23:35 [ERRO] Querying witness 0 failed: backend failure: rpc error: code = InvalidArgument desc = GetConsistencyProofRequest.FirstTreeSize: 0, want > 0