ensure dataframe updates as expected

[pngwn]

Fixes #890. Fixes #921.

(I think these are basically duplicates)

[pngwn]

Looking at the tests, it is a bit weird.

[abidlabs]

Were you able to figure out the tests @pngwn? It seems like it's affecting all of the open PRs

[abidlabs]

Still running into some issues with the Dataframe component. When I run either of these demos:

• demo\filter_records\run.py
• demo\fraud_detector\run.py
• demo\matrix_transpose\run.py

I get the following error in the javascript console when I hit submit:

TypeError: l.map is not a function

This issue prevent me from being able to test if #921 is fixed or not

[aliabid94]

btw we want to replace the "null"s with empty space right?

[abidlabs]

Yeah we should. That's a fix in the backend from None to ""

[abidlabs]

Created #987 based off of this branch

[abidlabs]

LGTM!

[abidlabs]

@pngwn would you like to merge dataframe-nulls into this branch and then merge both into blocks-dev?