fix(blooms): Fix tenants slice on loadTenantTables (backport k227)

[loki-gh-app]

Backport a8f7851 from #14900

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What this PR does / why we need it:

Followup for #14770. There were still empty tenants. Due to how we initialise and populate the slice.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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