[API Design] LinkHandler.getLink: Optional<Link> redundant to Link.isValid

[stefanseifert]

that is redundant and we should use only one of the concepts:

• having the linkhandler returning an empty optional element to signal an invalid link, removing the isValid method
• or having the linkhandler return and link without optional that is never null, and that can be check via isValid method

furthermore the model interfaces all just define a @Nullable Link - unclear whether invalid means null, or not null with isValid=false

with the current state it's quite confusing. there is already code where links are consumed always checking both !=null && isValid which is very ugly.

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this discussion is taken over from the mailing list where it did not gather any attention (except from @ky940819).