Hide dropdown menu then other networks list is empty

[saneery]

#2132

Motivation

Then SUPPORTED_CHAINS is [], we should hide the dropdown menu on the header

Before

After

[coveralls]

Pull Request Test Coverage Report for Build 24b26f09-a6ef-4843-97a8-3896384ee690

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 9 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.1%) to 80.715%

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Files with Coverage Reduction	New Missed Lines	%
apps/indexer/lib/indexer/fetcher/token.ex	1	78.57%
apps/indexer/lib/indexer/fetcher/token_balance.ex	2	87.1%
apps/indexer/lib/indexer/fetcher/internal_transaction.ex	3	58.62%
apps/indexer/lib/indexer/block/fetcher.ex	3	88.0%

Totals
Change from base Build 2e95c516-1e18-48e1-90e9-f00ed0466d7a:	-0.1%
Covered Lines:	4901
Relevant Lines:	6072

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💛 - Coveralls