"bbox" : [-180.0, -90.0, 180.0, 90.0],
it is definitely 'by design'. see com.esri.gpt.control.georss getValidMinX-like methods. perhaps something to change. there is not such a thing as a 'null geometry' from what I can see on GeoRSS, so better would be to omit the entire georss:polygon element when there is no geometry in a record.
if this happens with ArcGIS items, check the ags-toDC.xslt as there is a default extent assigned there upon publishing.