N/A instead of 0 for market cap if it is not fetched

[vbaranov]

Motivation

If TokenBridge MarketCap can not be calculated for some reason Blockscout displays 0, what can confuse users. We should display "N/A" instead.

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 4786beb1bf71c52bb1a1b914ed8261cce8a99b99-PR-3405

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+3.0%) to 67.512%

---

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

Totals
Change from base Build fc3c60ce3987ad1a5e51e696c94ffe376131c5c1:	3.0%
Covered Lines:	2049
Relevant Lines:	3035

---

💛 - Coveralls