Add UNSPECIFIED to BigDecimalRepresentation

[snicoll]

#5037 has changed the default for BigDecimal so that users have to provide an explicit feature if they want to map BigDecimal properties. I've noticed that the default for UuidRepresentation is UNSPECIFIED. This makes it explicit that it doesn't have a value (and the problem that could happen if not set).

It'd be nice if the BigDecimal representation had a similar signal. This would also align the default for those properties in Spring Boot.

[christophstrobl]

will do, thanks @snicoll.