--------------------------
	Allow data type declarations
	to have zero constructors
	--------------------------

This allows

	data T a

as a data type declaration; i.e. allows zero constructors.
If there is an '=' sign there must be at least one constructor.


* Parser.y: parse the declaration
* HsDecls: print out the data type declaration right
* TyCon: don't ASSERT that the constructors are non-empty