GeoJSON: don't use the name/title property as the object name if its value is null

I ran into some GeoJSON (from WFS server http://data.gov.au/geoserver/ows, Buildings layer, if you're interested) that has a null `name` property.  GeoJsonDataSource would use this property value as the object's name.  This may not be a huge problem in general, but in the `dataSourceBrowser` branch, `DataSourceItemViewModel` passes the name through `defaultValue`, which only checks for undefined and so still returns null, and then crashes when it calls `.replace` on that null.  Rather than fix `DataSourceItemViewModel`, I assumed that a null name would cause other problems in Cesium.  Plus, we'd probably like to fall back on the `title` property if one exists rather than using the null name property.  So my fix here is to avoid using `name` or `title` as the object name of it is falsy in any way.

Yeah, there are more words in this explanation than in the actual change.