CORE-6190 Fix for blank, hidden output filenames.

Updated metadactyl.analyses.common/build-default-values-map to also remove blank string values from its results, which prevents blank output filenames in JEX submissions.
Updated metadactyl.validation/validate-parameter to check for blank strings in hidden output default values.
Also updated metadactyl.zoidberg.app-edit/update-app-parameter to not save blank strings as default values.