7023034: Files.createTempDirectory((Path)null, "temp") does not throw NPE

[Alan Bateman]

A small oversight in the implementation of Files.createTempFile and 
createTempDirectory. If the temporary directory is specified as null 
then NPE should be thrown. The webrev with the changes is here:
  http://cr.openjdk.java.net/~alanb/7023034/webrev/

Thanks,
Alan.