Switch to using an EXISTS query

What does this MR do and why?

Before:

SELECT
    "security_findings".*
FROM
    "security_findings"
    INNER JOIN "security_scans" ON "security_findings"."scan_id" = "security_scans"."id"
WHERE
    "security_scans"."pipeline_id" = 832983315
LIMIT 1

• Current: hits: 65 (~520.00 KiB) from the buffer pool

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/17711/commands/59142

After:

SELECT 1 AS one 
FROM "security_findings" 
INNER JOIN "security_scans" ON "security_findings"."scan_id" = "security_scans"."id" 
WHERE "security_scans"."pipeline_id" = 600 AND (finding_data <> '{}') 
LIMIT 1;

• EXISTS: hits: 7 (~56.00 KiB) from the buffer pool

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/17725/commands/59158

• https://gitlab.com/gitlab-com/sec-sub-department/section-sec-request-for-help/-/issues/29
• !117290 (comment 1351848607)

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.