https://github.com/HUPO-PSI/mzTab/blob/master/specification_document-releases/2_0-Metabolomics-Release/mzTab_format_specification_2_0-M_release.adoc#6234-assay1-n states that an assay name is mandatory. For the https://github.com/HUPO-PSI/mzTab/blob/master/examples/2_0-Metabolomics_Release/MouseLiver_negative.mzTab example, assay names are not set, but validation is successful.