[SofaCore] Link: case where Data parent is invalid

[alxbilger]

parentData == nullptr leads to a crash. I think it happens when the linked object could not be created for some reasons.

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[alxbilger]

@damienmarchal I would like to know your opinion about what to do in the case parentData==nullptr, because I am not sure about what I did.

[alxbilger]

I tested different scenarios:
If ComponentA is defined before ComponentB:

1. A Data of ComponentA referring to a Data of ComponentB is not linked properly: the numeric value is not the right one, and Data that depends on the first Data are not updated accordingly.
2. A Data of ComponentB referring to a Data that does not exist (and will not exist) does not warn the user. The link is created (pointing to nothing) with a default numeric value.

Therefore, I suggest to restore the else statement with a warning to the user that the link does not have a parent. It seems that the case when a Data points to a Data that will be initialized later is not supported.
What do you think @damienmarchal ?