check from nullable to non-nullable works well

[Khatskevich]

check from nullable to non-nullable works well

[Khatskevich]

This behavior is prohibited.
Only non-null -> null evolution is allowed.

Reason: in this case we do not differentiate absence of a field and the field which is initialized with NULL.
If NULL was set intentionally, it cannot just be overridden to a value.

[Khatskevich]

This case is a little strange. Default values are not used if the
target schema is nullable and they are starting to be substituted
in case target schema becomes non-nullable.

Let user fix their data by hand if they really need this behavior.