[morph-developer] TextConverter using null as destinationClass

[Matt B. <gud...@ya...>]

I have noticed that TextConverter declares it is able
to use null as a source and destination class, but it
is actually not able to use null for a destination. 
And since lots of converters use a TextConverter to
provide for free multiple destination types with the
implementor only having to provide a String
conversion, these TextConverter consumers inherit the
problem.  This leaves a few options:

1. Agree that in the common paradigm converters allow
null sources but not destinations.  Because of the
NullConverter the majority of existing functionality
would be preserved.  The most unfortunate drawback of
this approach is that converters which declare a
single set of source-AND-destination types including
null (e.g. TextConverter) would have to be adjusted to
differ their listing of provided source/dest types.

2.  Alter the default isTransformableImpl behavior in
BaseTransformer (either here or in TransformerUtils)
to disallow null destinationClass.  Not sure I like
this either.

3.  Rework classes that declare null as a destination
type to actually be able to return null.  Most likely
here it would make sense to check for null
destinationType in BaseTransformer.convert()--I think
we can agree that there is only one possible return
value from a null conversion.  I would probably choose
this alternative, personally.

In any event, we also need to determine what the
expected behavior is of TextConverter when converting
null to a text type.  Is it empty, or is it "null"? 
Which is more likely to serve 99% of what users would
want OOTB?

WDYT?

-Matt B

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