Several NPE's in datatransfer were eliminated causing JCK failures

Problem: Several methods in the package java.awt.datatransfer -- primarily
methods of the class DataFlavor -- throw NPEs in responsed to a null argument.
We feel that in several of these places it would be more appropriate to
generate a more useful exception or assume a default behavior. In a few of
these places, the behavior of the method has already been changed in kestrel,
leading to JCK failures. However, rather than revert to the old behavior, we
would prefer to change the specifications of the methods and update the JCK
tests.

The testcases to be modified not to expect NPEs are:

DataFlavor0117, StringSelection0003, StringSelection0007,
UnsupportedFlavorException0002.