Display upper border for tx speed if the value cannot be calculated

[vbaranov]

Fixes #3072

Changelog

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 163303d7-8ed9-48ca-8d52-de2397f92cb5

• 2 of 4 (50.0%) changed or added relevant lines in 1 file are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.07%) to 75.072%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/block_scout_web/lib/block_scout_web/views/transaction_view.ex	2	4	50.0%

Files with Coverage Reduction	New Missed Lines	%
apps/block_scout_web/lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	1	78.57%
apps/block_scout_web/lib/block_scout_web/checksum_address.ex	6	55.56%

Totals
Change from base Build 547513a4-ec04-4fc6-9d77-bc844312e015:	-0.07%
Covered Lines:	5707
Relevant Lines:	7602

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💛 - Coveralls