Picard currently does not allow keeping empty strings in a multi-value variable, and automatically strips them out.  This can lead to problems for example if the user is trying to manage a list of artist join phrases, where the resulting multi-value may have elements removed so that it no longer matches the metadata.  This is the case with the changes to the AdditionalArtistsVariables plugin in PICARD-PLUGINS PR# 320 requiring a non-blank placeholder to be substituted which would have to be removed by the user when processing the variable in a script.