ensure no NULL is passed to memcpy

NULL is valid when size is 0, but memcpy uses nonnull attributes,
so let's not poke the bear

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -1077,11 +1077,11 @@ void BlockchainLMDB::add_tx_amount_output_indices(const uint64_t tx_id,
 
   int result = 0;
 
-  int num_outputs = amount_output_indices.size();
+  size_t num_outputs = amount_output_indices.size();
 
   MDB_val_set(k_tx_id, tx_id);
   MDB_val v;
-  v.mv_data = (void *)amount_output_indices.data();
+  v.mv_data = num_outputs ? (void *)amount_output_indices.data() : (void*)"";
   v.mv_size = sizeof(uint64_t) * num_outputs;
   // LOG_PRINT_L1("tx_outputs[tx_hash] size: " << v.mv_size);