Add null checks and public access modifier

[Falydoor]

Fix #13471

I'm not too familiar with reactor so let me know if there is a better way to handle this null check:

        if (persistentEntity == null) {
            throw new IllegalArgumentException("Unable to retrieve persisted entity");
        }

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[murdos]

@Falydoor : could you resolve the conflicts?

[Falydoor]

@murdos: Sonar check after resolving the conflicts

[murdos]

@Falydoor : thanks :)

[Falydoor]

Bounty claimed https://opencollective.com/generator-jhipster/expenses/34246

[pascalgrimaud]

@Falydoor : approved