In both Discover and TSVB you should be able to use a filter to filter those documents out:
the KQL query yourFieldName : *filters down to the documents that have the field yourFieldName set. I'm not sure whether the field is actually empty in your case or whether it just contains a - symbol. In the latter case you can filter it out using NOT yourFieldName: '-'
In TSVB you can set the filter in the Options tab of the series:
