[bugfix] Return empty result rather than 500 error when searching for blocked domains

[tsmethurst]

Description

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This pull request adds a quick domain block check in the search function of the processor to ensure that items being searched aren't currently domain blocked.

When we do #1497 we should consider removing these checks and doing them in the dereferencer instead.

closes #1462

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.