improve assertion failure if classpathAppend type is wrong;  as a convenience, add automatic elimination of empty/None classpath entries (since sometimes it's easier to add None to a list than to pass an empty list)
improve assertion failure if classpathAppend type is wrong; 
as a convenience, add automatic elimination of empty/None classpath entries (since sometimes it's easier to add None to a list than to pass an empty list)

xpybuild/xpybuild,xpybuild/xpybuild,xpybuild/xpybuild,xpybuild/xpybuild