UX follow-ups from "Resolve "[Frontend] Allow filtering compliance violations by branch"

What does this MR do and why?

Two minor UX fixes for violations report:

• replace debounced input with search box
• add detailed empty error message

Screenshots or screen recordings

Compliance_report___Commit451___GitLab__3_

How to set up and validate locally

1. Ensure you have valid EE license
2. Run FILTER=compliance_report_violations rake db:seed_fu to seed compliance report violations
3. Open /groups/Commit451/-/security/compliance_dashboard/violations in your GDK (assuming you have default Commit451 group)
4. Observe Search target branch filter using search box, not input
5. Enter anything except master to target branch filter
6. Observe detailed empty message
7. Reset target branch filter, select dates 1 month in the past
8. Observe simple empty message

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #407892 (closed)