[10.x] Make sure pivot model has previously defined values

[iamgergo]

This is a retry of #46522, however I try to be more clear about the issue here, also added tests with another approach.

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Due to Taylor's comment, my previous PR was closed, because the withPivotValue was mixed up with the withPivot method, however they are two diffent methods. (Also, as I see, the withPivotValue is not even documented, so if you wish, I gladly open a PR to document it, because I find it super handy.)

The withPivotValue defines a wherePivot statement and sets a pivot value when saving the relation. So it is used when saving AND retreiving relations.

The withPivot method just retrieves a pivot column, it does not even define a where statement.

When using the withPivotValue, that value is not being set when making a new pivot model using the relationship itself ($model->relation()->newPivot()).

It means, if a developer is using a custom pivot model, and that model has default attribute values using the $attributes array, when saving the relationship, the withPivotValue won't work, however it should override the proper attribute in the $attributes array.