Omit WHERE clause for NOT labels filter when empty

What does this MR do and why?

This clause will always be true when the labels provided are non-existent so we just remove it.

In certain cases (group issues list / boards), having this SQL clause results in PG picking a slow query plan.

Sample slow query: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7086/commands/25059

Time: 28.574 s
  - planning: 10.609 ms
  - execution: 28.563 s
    - I/O read: 2.312 s
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 42752481 (~326.20 GiB) from the buffer pool
  - reads: 2783 (~21.70 MiB) from the OS file cache, including disk I/O
  - dirtied: 196 (~1.50 MiB)
  - writes: 0

With the useless clause removed: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/7086/commands/25064

Time: 71.408 ms
  - planning: 6.458 ms
  - execution: 64.950 ms
    - I/O read: 0.000 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 41736 (~326.10 MiB) from the buffer pool
  - reads: 0 from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

That is from https://gitlab.com/groups/gitlab-org/-/issues?label_name%5B%5D=section%3A%3Adev&label_name%5B%5D=security&not%5Blabel_name%5D%5B%5D=xxx.

Related to #344074 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. In an issue list / board, add a NOT filter for a label that does not exist.
2. Check the query in the performance bar.

Before:

SELECT "issues".*
FROM "issues"
WHERE "issues"."project_id" = 278964
  AND ("issues"."state_id" IN (1))
  AND NOT (EXISTS (SELECT "label_links".* FROM "label_links" WHERE "label_links"."target_type" = 'Issue' AND "label_links"."target_id" = "issues"."id" AND 1=0))
  AND "issues"."issue_type" IN (0, 1)
ORDER BY "issues"."created_at" DESC, "issues"."id" DESC
LIMIT 20 OFFSET 0

After:

SELECT "issues".*
FROM "issues"
WHERE "issues"."project_id" = 278964
  AND ("issues"."state_id" IN (1))
  AND "issues"."issue_type" IN (0, 1)
ORDER BY "issues"."created_at" DESC, "issues"."id" DESC
LIMIT 20 OFFSET 0

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