Validator should issue an error if study_variable[1] is undefined but study_variable[1]-assay_refs is missing

[nilshoffmann]

This was discovered during import into LipidCompass. Unfortunately, this causes a NullPointerException, stopping the import of an otherwise valid file.

We should add this to the validator as an additional cross-check rule and should consider to automatically add the missing information when creating a new mzTab-M file.