Coinprice api endpoint: handle nil rates

[vbaranov]

Motivation

2021-03-21T15:36:54.476 application=block_scout_web request_id=Fm5llsNt_47yDO4AAEYE [error] Error while calling RPC action"** (Plug.Conn.WrapperError) ** (UndefinedFunctionError) function nil.last_updated/0 is undefined\n nil.last_updated()\n (block_scout_web 0.0.1) lib/block_scout_web/views/api/rpc/stats_view.ex:31: BlockScoutWeb.API.RPC.StatsView.prepare_rates/1\n (block_scout_web 0.0.1) lib/block_scout_web/views/api/rpc/stats_view.ex:23: BlockScoutWeb.API.RPC.StatsView.render/2\n (phoenix 1.5.6) lib/phoenix/view.ex:472: Phoenix.View.render_to_iodata/3\n (phoenix 1.5.6) lib/phoenix/controller.ex:776: Phoenix.Controller.render_and_send/4\n (block_scout_web 0.0.1) lib/block_scout_web/controllers/api/rpc/stats_controller.ex:1: BlockScoutWeb.API.RPC.StatsController.action/2\n (block_scout_web 0.0.1) lib/block_scout_web/controllers/api/rpc/stats_controller.ex:1: BlockScoutWeb.API.RPC.StatsController.phoenix_controller_pipeline/2\n (block_scout_web 0.0.1) lib/block_scout_web/controllers/api/rpc/rpc_translator.ex:101: BlockScoutWeb.API.RPC.RPCTranslator.call_controller/3\n"

Changelog

Check rates are not null object before return its data in coinprice API endpoint

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 32db2883ca1700473b1012a64c0e9937ce1e6c4f-PR-3728

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.4%) to 64.641%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/views/api/rpc/stats_view.ex	1	75.0%
test/block_scout_web/features/pages/chain_page.ex	1	90.0%

Totals
Change from base Build 6c56cbde1e195446c570035d10ec2d4ea01034b1:	0.4%
Covered Lines:	2117
Relevant Lines:	3275

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💛 - Coveralls