Get token type for token balance update if it is empty

[vbaranov]

Motivation

An error of such kind appears when indexing token balances:
application=indexer count=200 error_count=200 [debug] failed to import token balances: [#Ecto.Changeset<action: nil, changes: %{address_hash: %Explorer.Chain.Hash{byte_count: 20, bytes: <<91, 180, 86, 205, 9, 216, 81, 86, 225, 130, 210, 199, 121, 126, 180, 154, 67, 132, 1, 135>>}, block_number: 10963708, token_contract_address_hash: %Explorer.Chain.Hash{byte_count: 20, bytes: <<192, 42, 170, 57, 178, 35, 254, 141, 10, 14, 92, 79, 39, 234, 217, 8, 60, 117, 108, 194>>}, value: #Decimal<1>, value_fetched_at: ~U[2021-10-24 20:29:17.692398Z]}, errors: [token_type: {"can't be blank", [validation: :required]}]

Changelog

Check if all items in token balances changeset contain token type and if some of the items don't have a token type, fetch it from tokens table

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