Add a guard that addresses is enum in values function in read contract page

[vbaranov]

#2907

Motivation

Read page is broken if addresses output is not a list

Changelog

Check that addresses type is an enum

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 db62c813-c7f7-493b-8f91-64bcdd5a830e

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 75.248%

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Files with Coverage Reduction	New Missed Lines	%
apps/explorer/lib/explorer/chain_spec/genesis_data.ex	5	9.09%

Totals
Change from base Build 10acea3e-75cc-4479-9c48-3d6a58431711:	-0.07%
Covered Lines:	5378
Relevant Lines:	7147

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💛 - Coveralls