Fix withdrawals sum count on empty database

[sl1depengwyn]

Motivation

2023-05-25T17:34:07.299 [error] GenServer Explorer.Chain.Cache.WithdrawalsSum terminating
** (FunctionClauseError) no function clause matching in Explorer.Chain.Wei.to/2
    (explorer 5.1.5) lib/explorer/chain/wei.ex:251: Explorer.Chain.Wei.to(nil, :wei)
    (explorer 5.1.5) lib/explorer/chain/cache/withdrawals_sum.ex:72: Explorer.Chain.Cache.WithdrawalsSum.consolidate/0
    (explorer 5.1.5) lib/explorer/chain/cache/withdrawals_sum.ex:38: Explorer.Chain.Cache.WithdrawalsSum.handle_continue/2
    (stdlib 5.0) gen_server.erl:1067: :gen_server.try_handle_continue/3
    (stdlib 5.0) gen_server.erl:977: :gen_server.loop/7
    (stdlib 5.0) proc_lib.erl:241: :proc_lib.init_p_do_apply/3
Last message: {:continue, :ok}
State: %{consolidate?: true}

Changelog

Add handling of nil response from aggregate query

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools