Add option to support null as default value

When the option `defaultNull` is set and no default value is giving for
a missing field `null` is used instead of the type specific default.

Add option to support null as default value

When the option `defaultNull` is set and no default value is giving for
a missing field `null` is used instead of the type specific default.

3.2.1

3.2.0

3.1.8

3.1.7

3.1.6

3.1.5

3.1.4

3.1.3