Fix: skip relationship query when Repeater has no parent model

[FS-Baz]

Description

A Repeater with ->relationship() queries the related table on a Create page even though no parent model exists yet. getCachedExistingRecords() does not short-circuit when there is no model, so it runs select * from "posts" where "posts"."author_id" is null and "posts"."author_id" is not null — one redundant query per relationship Repeater per create-page render. Fixed by short-circuiting when there is no persisted model, mirroring EntanglesStateWithSingularRelationship which already guards with ! $record?->exists.

Reproduction: https://github.com/FS-Baz/filament-repeater-repro

Visual changes

None — internal behavior only, no UI change.

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.