Follow up for https://github.com/uwdata/vegalite-ui/pull/153 (filter null only for fields on the shelf)
Follow up for https://github.com/uwdata/vegalite-ui/pull/153 (filter null only for fields on the shelf)

dennari/hack-n-hackers-polestar,sandbox/polestar,vivekratnavel/polestar,pallavkul/polestar,sandbox/polestar,smartpcr/polestar,zegang/polestar,jsanch/polestar,vega/polestar,hortonworks/polestar,sandbox/polestar,jsanch/polestar,vega/polestar,vivekratnavel/polestar,vivekratnavel/polestar,pallavkul/polestar,smartpcr/polestar,uwdata/polestar,pallavkul/polestar,jsanch/polestar,dennari/hack-n-hackers-polestar,vega/polestar,zegang/polestar,uwdata/polestar,smartpcr/polestar,uwdata/polestar,hortonworks/polestar,dennari/hack-n-hackers-polestar,hortonworks/polestar,zegang/polestar