Effect of removing redundant nullptr checks in deleters

baseline commit, patch commit, filtered with --benchmark_filter="(shrink|delete).*unodb::db"

• micro_benchmark_node4: 2% slowdown (full_node4_to_minimal_sequential_delete<unodb::db>/65532) to 11% speedup (shrink_node16_to_node4_randomly<unodb::db>/64)
• micro_benchmark_node16: 3% slowdown (full_node16_tree_random_delete<unodb::db>/32768) to 7% speedup (shrink_node48_to_node16_randomly<unodb::db>/16383)
• micro_benchmark_node48: 3% slowdown (full_node48_tree_sequential_delete<unodb::db>/192) to 12% speedup (shrink_node256_to_node48_randomly<unodb::db>/64)
• micro_benchmark_node256: 1% slowdown