Transforms an ExternalNodeAssociation (IssueVersion) based on the project import mapper that is provided. This should only be
used with a fully mapped and validated ProjectImportMapper.
projectImportMapper - a fully mapped and validated ProjectImportMapper
version - the external node association that contains all the old values that need to be transformed and other values
that should be stored that need no transformation.
a new ExternalNodeAssociation that contains the transformed values based on the projectImportMapper. Null
if the node association type is not a version type or if the issue or version id's have not been mapped.