fix: don't panic when issuer wasn't found for vote

[pavelbrm]

Summary

This PR fixes a panic when the issuer wasn't found whilst voting.

There has been a spike in panics due to not found issuer because the datastore method returns nil, nil when not found, and the caller does not check the result for nil-ness.

Type of Change

• Product feature
• Bug fix
• Performance improvement
• Refactor
• Other

Tested Environments

• Development
• Staging
• Production

Before Requesting Review

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• Have you added tests for new functionality?
• Have validated query efficiency for new database queries?
• Have documented new functionality in README or in comments?
• Have you squashed all intermediate commits?
• Is there a clear title that explains what the PR does?
• Have you used intuitive function, variable and other naming?
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• Have you performed a self review of this PR?

Manual Test Plan