Make friction_value in CrossSectionLocation table nullable

[margrietpalm]

CrossSectionDefinition.friction_values can be present instead of CrossSectionLocation.friction_value. However, all nullable columns are automatically checked in https://github.com/nens/threedi-modelchecker/blob/master/threedi_modelchecker/config.py#L41. So to allow for the situation where CrossSectionLocation.friction_value is None and CrossSectionDefinition.friction_values is defined, CrossSectionLocation.friction_value needs to become nullable.

The changes in the modelchecker related to this are here: nens/threedi-modelchecker@70f9393

[elisalle]

Haven't tried running the new migration but it looks good. Just the changelog still needs updating and it should be good to go.

[margrietpalm]

Thanks @elisalle. Changelog is now updated