Manual exports should have an option to exclude nulls in the collector

Unsure what the best solution would be here.
* Doing it at the Collector level is another permutation of collectors to think about, but it give the control to the calling code and would mean that the collector doesn't need a `valueIfNull` argument.
* Could add an `exportSkipNulls(...)` method. This would make it easier to apply value transforms b/c there would be no nulls. Exotic Properties could have a case where the value is an empty string and `getValueAsString()` returns null, causing the need for null checks still. That would be a bad Property citizen.

Possibly update the Property and PropertyType docs to make it explicit that if `getValue()` returns null, `getValueAsString()` should return null as well, and visa versa.