go/vt/wrangler: add len(qr.Rows) check to no streams found log msg

[maxenglander]

Description

Very minor improvement to vexec_plan.go log message for when no matching streams are found.

Related Issue(s)

• Fixes: Bug Report: misleading vexec_plan.go log message "no matching streams found" #14060

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[mattlord]

Thanks, @maxenglander ! ❤️