Ksql: Support for `NON NULL` columns

[big-andy-coates]

At the moment, all columns in KSQL are implicitly NULLable. The SQL standard allows a column to be defined as NON NULL and this is a useful feature to ensure data integrity.

KSQL should support NON NULL columns.

It should also allow components of complex types to be non-null too, e.g. a MAP with non-null key or value parts, or a STRUCT with a mixture of nullable and non-null fields.

[big-andy-coates]

If we were to make MAP have non-null keys by default, then maps could be serialized as Avro maps, rather than arrays of key-value pairs, (which confuses a lot of people!)

[eshepelyuk]

Voting for the issue and interested which milestone it's gonna be targeted for ?