Token instance: check if external_url is not null before trimming

[vbaranov]

Motivation

Eliminate error like this

Request: GET /tokens/0x7D37700d88D5258e6Ca100D2694dD87A49101412/instance/3/token-transfers
** (exit) an exception was raised:
    ** (FunctionClauseError) no function clause matching in String.Break.trim_leading/1
        (elixir 1.10.3) lib/elixir/unicode/properties.ex:298: String.Break.trim_leading(nil)
        (elixir 1.10.3) lib/string.ex:1177: String.trim/1
        (block_scout_web 0.0.1) lib/block_scout_web/views/tokens/instance/overview_view.ex:51: BlockScoutWeb.Tokens.Instance.OverviewView.external_url/1
        (block_scout_web 0.0.1) lib/block_scout_web/templates/tokens/instance/overview/_details.html.eex:66: BlockScoutWeb.Tokens.Instance.OverviewView."_details.html"/1
        (block_scout_web 0.0.1) lib/block_scout_web/templates/tokens/instance/transfer/index.html.eex:2: BlockScoutWeb.Tokens.Instance.TransferView."index.html"/1
        (phoenix 1.5.4) lib/phoenix/view.ex:310: Phoenix.View.render_within/3
        (phoenix 1.5.4) lib/phoenix/view.ex:472: Phoenix.View.render_to_iodata/3
        (phoenix 1.5.4) lib/phoenix/controller.ex:776: Phoenix.Controller.render_and_send/4

Changelog

Check if external_url is nil in metadata before trimming

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

[coveralls]

Pull Request Test Coverage Report for Build 2f3eb5d93feb825fa20ad988e9d94f215bf12073-PR-3295

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+3.2%) to 75.836%

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Totals
Change from base Build 7be4f27d101f38ecffe8bfecf015887f8644b07d:	3.2%
Covered Lines:	1927
Relevant Lines:	2541

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💛 - Coveralls