mimir-mixin: Fix "unexpected type object, expected array"

[narqo]

What this PR does

This is a fixup for #7413

When show_rejected_requests_on_writes_dashboard is set in the config, the code path in writes.libjsonnet fails with

2024/02/23 19:48:38 RUNTIME ERROR: Unexpected type object, expected array
	operations/mimir-mixin/dashboards/writes.libsonnet:(139:20)-(151:22)
	During manifestation

This is due to how I originally misinterpreted the {} in the if C then {} as condition C's block:

TIL (1) the {} is an actual object, inserted into the resulting JSON.
TIL (2) jsonnet's linter doesn't catch a broken syntax, i.e. [] + {}, when this happens inside a dead code.

Note there are no assets to rebuild for the PR, because the mentioned config is unset in mimir. But the problem showed itself in the internal build.

Which issue(s) this PR fixes or relates to

Fixes n/a

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[narqo]

tl;dr; of these changes:

- if C { targets: [···] + {···} }
+ if C { targets: [···] } + {···}

[56quarters]

LGTM. Tested by building locally and confirming the changes here fix the error in main when show_rejected_requests_on_writes_dashboard: true