Add a null-check to java enum safeValueOf

[BoD]

Fix for #5901

• Also make the constructor of the class and of UNKNOWN__ nullable
• Also call super() in equals() and hashCode()

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[martinbonnin]

A few comments. Also, can we do the same for Kotlin codegen?

[BoD]

can we do the same for Kotlin codegen?

You're right! We can make the class constructor private. I'd keep the Kotlin UNKNOWN__ constructor public though as it's a nice way to signal a warning when using it, thanks to the opt-in annotation. On the other hand, it's still possible to call safeValueOf without any warning... Maybe safeValueOf should be annotated too?

[martinbonnin]

I'd keep the Kotlin UNKNOWN__ constructor public though as it's a nice way to signal a warning when using it, thanks to the opt-in annotation

I'd try to keep the symmetry with Java as much as possible.

On the other hand, it's still possible to call safeValueOf without any warning... Maybe safeValueOf should be annotated too?

I'd go for that. And rename the @ApolloEnumConstructor to @ApolloUnknownEnum or something like that.

[BoD]

Turns out, actually in Kotlin we can't make the UNKOWN__ constructor private, because unlike Java, outer classes can't access private constructors of inner classes. I kept the optin annotation there, and also applied it to safeValueOf.