Need isNull isNotNull type filter option

[darylerwin]

Make sure these boxes are checked before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if any
• I have reproduced the issue with at least the latest released version of superset
• I have checked the issue tracker for the same issue and I haven't found one similar

Superset version

0.24.0

Expected results

the generated query to allow entries like field1 is not null

Actual results

field1 != 'NULL'

Steps to reproduce

[darylerwin]

workaround is to use the custom WHERE box.

[hughhhh]

#4651

[SpyderRivera]

I am able to filter nulls with != and then not filling in the options box...

[darylerwin]

Closing as noted there is a PR to cover this issue. Thanks.

[tush4hworks]

what spyder said doesn't work for me, can you tell exactly what you're doing?

[mistercrunch]

In latest version there's a IS NULL and IS NOT NULL option