Implement NullFilter.

[blackwinter]

Some modules can't handle null values (e.g. FormetaEncoder and SimpleXmlEncoder), which (at least) MarcXmlHandler may generate (ref.). There are several options to deal with this situation:

1. Fix all modules to not insert null values into the stream.
2. Fix all modules to handle null values (as has happened with StreamUnicodeNormalizer, see Ignore null values in StreamUnicodeNormalizer. #262).
3. Add a new filter to discard or replace null values.

This pull request implements the latter option.

[cboehme]

I remember Markus and I discussing the issue a couple of years ago. I think, we decided that id and name parameters should never be null while value may be null. I am not sure if allowing null for values is a sensible choice. We should discuss this.

For now a NullFilter is good short-term remedy for modules emitting null values.

[blackwinter]

I am not sure if allowing null for values is a sensible choice.

I would think so. Ideally, all modules would handle null values gracefully (option 2); either deal with them appropriately (e.g., JsonEncoder) or ignore them/pass them through (e.g., StreamUnicodeNormalizer). But this looks like a daunting task ;)