reducer results should not contain `undefined`

[wmertens]

Since results are stored as JSON, undefined values aren't visible but they do remove keys from objects.

Therefore, a two-pronged breaking approach for v4

• do not allow undefined values in objects returned by reducers
• make a jsonColumn() column helper that removes null values from objects
• make a jsonWithNullColumn() column helper that keeps null values
• make the 'JSON' type behave like json()

[wmertens]

#24 includes a deprecation notice for reducers that return undefined values

[Dzieni]

deprecation notice done in #24