[Core] Fix isSet flag when setting a link to a not-yet created data

[fredroy]

In SimpleAPI and SofaPython3 (surely XML too),
if a data link is set to a data of a not-yet created BaseObject, isSet is not true.

In the newly created tests, the last one would be false without the fix in BaseData

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[fredroy]

[ci-build][with-all-tests]

[alxbilger]

Is this related to #4910?

[hugtalbot]

This should fix #4911

[fredroy]

Is this related to #4910?

Maybe but It was not my intention 😅 I got the problem in SofaPython3 for our own component.

This should fix #4911

It would have been convenient to have a test related to this problem 👼