Prevent update validator metadata with empty name from contract

[vbaranov]

Motivation

If validator's name is not from the metadata contract (hardcoded in DB), it wipes with empty name on the next scheduled update of metadata

Changelog

Do not update metadata if name in contract's is empty

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 7e5a2a40c9577b6a4bcd889f48090eb268895e03-PR-3600

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.6%) to 65.618%

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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%
test/block_scout_web/features/pages/chain_page.ex	1	90.0%

Totals
Change from base Build f9b34d2546520b5f11b829c29d15a78dfe89c1f3:	1.6%
Covered Lines:	2086
Relevant Lines:	3179

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💛 - Coveralls