Read contract: fix internal server error for method with empty output array

[vbaranov]

Motivation

 Request: GET /smart-contracts?hash=0x47ed323d03cd3fff423f1200af2e2e3d823e8d0a&type=regular&action=read
blockscout     | ** (exit) an exception was raised:
blockscout     |     ** (Protocol.UndefinedError) protocol Enumerable not implemented for "" of type BitString. This protocol is implemented for the following type(s): Indexer.BoundQueue, Ecto.Adapters.SQL.Stream, Postgrex.Stream, DBConnection.PrepareStream, DBConnection.Stream, Timex.Interval, Flow, Map, MapSet, Function, HashDict, IO.Stream, File.Stream, List, Stream, Range, HashSet, Date.Range, GenEvent.Stream
blockscout     |         (elixir 1.10.4) lib/enum.ex:1: Enumerable.impl_for!/1
blockscout     |         (elixir 1.10.4) lib/enum.ex:141: Enumerable.reduce/3
blockscout     |         (elixir 1.10.4) lib/enum.ex:3383: Enum.map/2
blockscout     |         (explorer 0.0.1) lib/explorer/smart_contract/reader.ex:449: Explorer.SmartContract.Reader.new_value/3
blockscout     |         (explorer 0.0.1) lib/explorer/smart_contract/reader.ex:424: anonymous fn/3 in Explorer.SmartContract.Reader.link_outputs_and_values/3
blockscout     |         (elixir 1.10.4) lib/enum.ex:2111: Enum."-reduce/3-lists^foldl/2-0-"/3
blockscout     |         (explorer 0.0.1) lib/explorer/smart_contract/reader.ex:423: Explorer.SmartContract.Reader.link_outputs_and_values/3
blockscout     |         (explorer 0.0.1) lib/explorer/smart_contract/reader.ex:313: Explorer.SmartContract.Reader.fetch_current_value_from_blockchain/3

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 cf659a196aaa82cc198080744b845989fe11beca-PR-3514

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.3%) to 67.023%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	1	78.57%

Totals
Change from base Build 19c744f1023b2c23cc8df0fb7216dcbd4e93bbb8:	-0.3%
Covered Lines:	2071
Relevant Lines:	3090

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💛 - Coveralls