fix(genjava.base): use Object for heterogeneous lists for the join type + support the empty type

[alexanderpann]

• both generators use Object as the type:
• fixes Java Generator for JoinType not supported #799

val mixed = list(true, 1, "two")                                          
                                                                          
test case mixedAccess [incomplete] {                                      
assert  mixed[0] equals true [C] [no result found]                        
assert  mixed[1] equals 1 [C] [no result found]                           
assert  mixed[2] equals "two" [C] [no result found]                       
assert  mixed.size equals 3 [C] [no result found]                         
assert  mixed.all(|it.toList.size == 1|) equals true [C] [no result found]
assert  mixed.any(|it.toList.size == 1|) equals true [C] [no result found]
assert  mixed.contains(true) equals true [C] [no result found]            
assert  mixed.first equals true [C] [no result found]                     
assert  mixed.last equals "two" [C] [no result found]                     
assert  mixed.indexOf(1) equals 1 [C] [no result found]                   
assert  mixed.reverse equals list("two", 1, true) [C] [no result found]   
assert  mixed.tail equals list(1, "two") [C] [no result found]            
}                                                                         

I think we should create a ticket for improving other operations on heterogeneous lists. An example of what doesn't work:

assert mixed.any(|it.oneOf[true, false]|) equals true -> type join(...) is not a subtype of boolean

@arimer Do you have more input on what might be not working?