ConfigKeyBuilder has the wrong default for defaultValue

[Claudenw]

What happened?

The ConfigKeyBuilder.defaultValue property has a default value of null. Based on the default value in the current ConfigKey constructor the default value should be ConfigDef.NO_DEFAULT_VALUE

What did you expect to happen?

When not defined the defaultValue of the resulting key should be NO_DEFAULT_VALUE.

[Claudenw]

This change will require a new major version as it will break versions already in the field.