Transaction actions fix

[varasev]

Motivation

We had the following error when handling nil log.first_topic:

2023-02-07 12:34:31	
{"log":"    Args: []\n","stream":"stdout","time":"2023-02-07T09:34:31.242592344Z"}
2023-02-07 12:34:31	
{"log":"Function: #Function\u003c2.49978846/0 in Indexer.Fetcher.TransactionAction.handle_info/2\u003e\n","stream":"stdout","time":"2023-02-07T09:34:31.242590148Z"}
2023-02-07 12:34:31	
{"log":"    (indexer 5.0.0) lib/indexer/fetcher/transaction_action.ex:115: Indexer.Fetcher.TransactionAction.task/1\n","stream":"stdout","time":"2023-02-07T09:34:31.242587972Z"}
2023-02-07 12:34:31	
{"log":"    (elixir 1.13.1) lib/enum.ex:4136: Enum.reduce_range/5\n","stream":"stdout","time":"2023-02-07T09:34:31.242586044Z"}
2023-02-07 12:34:31	
{"log":"    (indexer 5.0.0) lib/indexer/fetcher/transaction_action.ex:126: anonymous fn/7 in Indexer.Fetcher.TransactionAction.task/1\n","stream":"stdout","time":"2023-02-07T09:34:31.242583758Z"}
2023-02-07 12:34:31	
{"log":"    (indexer 5.0.0) lib/indexer/transform/transaction_actions.ex:127: Indexer.Transform.TransactionActions.parse/2\n","stream":"stdout","time":"2023-02-07T09:34:31.242581543Z"}
2023-02-07 12:34:31	
{"log":"    (elixir 1.13.1) lib/enum.ex:4035: Enum.filter_list/2\n","stream":"stdout","time":"2023-02-07T09:34:31.242578989Z"}
2023-02-07 12:34:31	
{"log":"    (elixir 1.13.1) lib/enum.ex:4034: Enum.filter_list/2\n","stream":"stdout","time":"2023-02-07T09:34:31.242576728Z"}
2023-02-07 12:34:31	
{"log":"    (indexer 5.0.0) lib/indexer/transform/transaction_actions.ex:167: anonymous fn/1 in Indexer.Transform.TransactionActions.uniswap_filter_logs/1\n","stream":"stdout","time":"2023-02-07T09:34:31.242571503Z"}
2023-02-07 12:34:31	
{"log":"    (elixir 1.13.1) lib/string.ex:877: String.downcase(nil, :default)\n","stream":"stdout","time":"2023-02-07T09:34:31.242569161Z"}
2023-02-07 12:34:31	
{"log":"** (FunctionClauseError) no function clause matching in String.downcase/2\n","stream":"stdout","time":"2023-02-07T09:34:31.24256636Z"}

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