Support Generic nodes with no type variables

[HertzDevil]

Resolves #10204.

This is purely a syntax enhancement and requires no changes outside the parser. It is already possible to obtain generic instances that have no type variables even before this PR, such as via Tuple.new; even for Generic nodes, all instantiations of NamedTuple return an empty array in #type_vars. (In contrast, resolve.type_vars returns a 1-array of the NamedTuple or Tuple itself.)

At the lexical level, a constant followed by a pair of empty parentheses can currently appear only in the following places:

lib Foo
  fun Foo() # lib fun definition
end

Foo.Foo() # lib fun call

fun bar = Bar() # global fun definition
end

annotation Ann; end

@[Ann()] # annotation with no arguments
def ann; end

The formatter removes the parentheses in all these cases. However, if we look at all empty parentheses in general, then we can see that this is not done for Calls because they could be incorrectly turned into Vars. In this PR the formatter never removes the parentheses in a Generic without type variables. Also note that in the following example:

class Foo(); end # Error: must specify at least one type var

the compiler never materializes any Generic, because the parser handles this separately in #parse_type_vars and the corresponding ClassDef / ModuleDef stores the formal parameters outside the type name.