Empty tokens produce invalid lexer specifications

[bringert]

If there are empty tokens in the LBNF file, several of the back-ends produce invalid lexer specifications. For example:

F. C ::= "" ;
G. C ::= "foo" ;

At least the generated Alex and JLex code for this is invalid.

BNFC should consider empty tokens to be errors, or possibly silently convert them to empty tokens lists.

This issue was first reported by didier.garcin@…