Fix for empty results when no shards can be found to route to [v3]

[systay]

Description

Queries that follow the following pattern -

select exists(select * from foo where some_field=some_value)

The table foo would has a lookup vindex on some_field and there are no matching rows for some_value. What Vitess would do when no shards could be found to route to is to return an empty result. This is correct for most cases, but not always. The query above would return an empty result instead of a 0.

This fix is for the V3 planner.

Related Issue(s)

Fixes #10144
Forward port of #10202

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

[github-actions]

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[systay]

will re-open once #10202 is done