Almost stop using marshmallow defaults (except Nones)

[roman-anna-money]

Due to fix of a typo in tests, it has been found that complex default values don't work for marshmallow2 (it tries to parse a default value via field), but do work for marshmallow3: test_nested_default demonstrates the issue for a dataclass field default.

So, as a solution of the issue, I propose to stop settings default values to marshmallow (except Nones for Optional fields, because our schema removes Nones by default).

Known side effects:

1. a default value for such a field will not be shown in Swagger, however it will be not required there.