Check if exists custom_cap property of extended token object before access it

[vbaranov]

Motivation

Request: GET /token/0x431ad2ff6a9C365805eBaD47Ee021148d6f7DBe0/token-transfers
** (exit) an exception was raised:
    ** (KeyError) key :custom_cap not found in: %{__meta__: #Ecto.Schema.Metadata<:loaded, "tokens">, __struct__: Explorer.Chain.Token, bridged: nil, cataloged: true, contract_address: %Explorer.Chain.Address{__meta__: #Ecto.Schema.Metadata<:loaded, "addresses">, contract_code: %Explorer.Chain.Data{bytes: <<96, 128, 96, 64, 82, 52, 128, 21, 97, 0, 16, 87, 96, 0, 128, 253, 91, 80, 96, 4, 54, 16, 97, 1, 95, 87, 96, 0, 53, 124, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...>>}, contracts_creation_internal_transaction: #Ecto.Association.NotLoaded<association :contracts_creation_internal_transaction is not loaded>, contracts_creation_transaction: #Ecto.Association.NotLoaded<association :contracts_creation_transaction is not loaded>, decompiled: true, decompiled_smart_contracts: #Ecto.Association.NotLoaded<association :decompiled_smart_contracts is not loaded>, fetched_coin_balance: #Explorer.Chain.Wei<0>, fetched_coin_balance_block_number: 13488030, has_decompiled_code?: nil, hash: %Explorer.Chain.Hash{byte_count: 20, bytes: <<67, 26, 210, 255, 106, 156, 54, 88, 5, 235, 173, 71, 238, 2, 17, 72, 214, 247, 219, 224>>}, inserted_at: ~U[2019-07-25 02:10:45.349094Z], names: #Ecto.Association.NotLoaded<association :names is not loaded>, nonce: nil, smart_contract: nil, smart_contract_additional_sources: #Ecto.Association.NotLoaded<association :smart_contract_additional_sources is not loaded>, stale?: nil, token: #Ecto.Association.NotLoaded<association :token is not loaded>, updated_at: ~U[2019-07-25 02:10:45.349094Z], verified: false}, contract_address_hash: %Explorer.Chain.Hash{byte_count: 20, bytes: <<67, 26, 210, 255, 106, 156, 54, 88, 5, 235, 173, 71, 238, 2, 17, 72, 214, 247, 219, 224>>}, decimals: #Decimal<18>, holder_count: 1652, inserted_at: ~U[2019-07-25 03:39:28.587232Z], name: "dForce", skip_metadata: nil, symbol: "DF", total_supply: #Decimal<999934997348875914379213820>, type: "ERC-20", updated_at: ~U[2021-10-24 10:42:38.416602Z], usd_value: #Decimal<0.216994>}
        (block_scout_web 0.0.1) lib/block_scout_web/views/tokens/overview_view.ex:62: BlockScoutWeb.Tokens.OverviewView.total_supply_usd/1
        (block_scout_web 0.0.1) lib/block_scout_web/templates/tokens/overview/_details.html.eex:108: BlockScoutWeb.Tokens.OverviewView."_details.html"/1
        (block_scout_web 0.0.1) lib/block_scout_web/templates/tokens/transfer/index.html.eex:2: BlockScoutWeb.Tokens.TransferView."index.html"/1
        (phoenix 1.5.13) lib/phoenix/view.ex:310: Phoenix.View.render_within/3
        (phoenix 1.5.13) lib/phoenix/view.ex:472: Phoenix.View.render_to_iodata/3
        (phoenix 1.5.13) lib/phoenix/controller.ex:777: Phoenix.Controller.render_and_send/4
        (block_scout_web 0.0.1) lib/block_scout_web/controllers/tokens/transfer_controller.ex:1: BlockScoutWeb.Tokens.TransferController.action/2
        (block_scout_web 0.0.1) lib/block_scout_web/controllers/tokens/transfer_controller.ex:1: BlockScoutWeb.Tokens.TransferController.phoenix_controller_pipeline/2

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