skip cache update if there are no blocks inserted

[ayrat555]

Sometimes due to some error (mostly network) we may
not fetch any data when indexing block. If block cache update
is triggered it fails.

This PR checks if list is passed to cache update function.

Changelog

• skip cache update if there are no blocks inserted

[coveralls]

Pull Request Test Coverage Report for Build d6cedeb7-7914-4c58-9688-4599c2207b77

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 180 unchanged lines in 22 files lost coverage.
• Overall coverage decreased (-2.5%) to 76.225%

---

Files with Coverage Reduction	New Missed Lines	%
apps/ethereum_jsonrpc/lib/ethereum_jsonrpc/http.ex	1	74.0%
apps/ethereum_jsonrpc/test/support/ethereum_jsonrpc/case/parity/http_websocket.ex	1	0.0%
apps/ethereum_jsonrpc/test/support/ethereum_jsonrpc/case/parity/mox.ex	1	0.0%
apps/indexer/lib/indexer/buffered_task.ex	1	85.07%
apps/indexer/lib/indexer/fetcher/staking_pools.ex	1	75.0%
apps/indexer/test/support/indexer/block/catchup_supervisor_case.ex	1	0.0%
apps/ethereum_jsonrpc/lib/ethereum_jsonrpc/parity/traces.ex	2	0.0%
apps/indexer/test/support/indexer/fetcher/pending_transaction_supervisor_case.ex	2	0.0%
apps/ethereum_jsonrpc/lib/ethereum_jsonrpc.ex	3	80.0%
apps/ethereum_jsonrpc/test/support/ethereum_jsonrpc/web_socket/case/parity.ex	4	0.0%

Totals
Change from base Build c31ac9df-9686-4cbb-86da-b46b40fe0eba:	-2.5%
Covered Lines:	5011
Relevant Lines:	6574

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💛 - Coveralls