Fix default image tag type in values

[Blamas]

What was changed

Fix the default type of the image tag from map {} to string "" in the values file. Issue introduced in https://github.com/temporalio/helm-charts/pull/488

Why?

It's causing these errors when rendering the chart

coalesce.go:286: warning: cannot overwrite table with non table for temporal.temporal.server.image.tag (map[])

Checklist

1. Closes None

2. How was this tested: By rendering helm templates locally

3. Any docs updates needed? No

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[robholland]

Thanks :)