do not update cache if no blocks were inserted

[ayrat555]

fixes

2019-09-25T10:32:09.995 application=indexer fetcher=block_catchup first_block_number=8617628 last_block_number=8617628 [error] ** (Enum.EmptyError) empty error
    (elixir) lib/enum.ex:1721: anonymous fn/0 in Enum.min_max_by/2
    (indexer) lib/indexer/block/fetcher.ex:188: Indexer.Block.Fetcher.update_block_cache/1
    (indexer) lib/indexer/block/fetcher.ex:177: Indexer.Block.Fetcher.fetch_and_import_range/2
    (indexer) lib/indexer/block/catchup/fetcher.ex:194: Indexer.Block.Catchup.Fetcher.fetch_and_import_range_from_sequence/3
    (elixir) lib/task/supervised.ex:90: Task.Supervised.invoke_mfa/2
    (elixir) lib/task/supervised.ex:35: Task.Supervised.reply/5
    (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3

Changelog

• do not update cache if no blocks were inserted

[coveralls]

Pull Request Test Coverage Report for Build 08abf312-f1b3-47d3-92ff-fb99bda497ef

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.489%

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Totals
Change from base Build f778debe-ab2d-42cf-b671-d60e5a3436b3:	0.0%
Covered Lines:	5258
Relevant Lines:	6699

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💛 - Coveralls

[pasqu4le]

👍