Refine `QuerydslRepositoryInvokerAdapter` nullability assertions

[spring-projects-issues]

Oliver Drotbohm opened DATACMNS-1055 and commented

• QuerydslRepositoryInvokerAdapter should reject null Predicate instances on construction and thus rather force clients to not create an instance in the first place
• QuerydslPredicateBuilder.getPredicate(…) should return Optional<Predicate>

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Referenced from: pull request #294

[mp911de]

QuerydslPredicateBuilder.getPredicate(…) is no longer nullable, see #2396