Fix type names for generic instances with empty splat type vars

[HertzDevil]

Fixes an issue where if a generic type has at least one splat type variable and one non-splat type variable, and a generic instance has zero type arguments corresponding to the splat, that instance's type name includes an extra comma:

module Foo(A, *B)
  {% Foo(Int32) %} # => Foo(Int32, )
end

module Bar(*A, B)
  {% Bar(Int32) %} # => Bar(, Int32)
end

module Baz(A, *B, C)
  {% Baz(Int32, String) %} # => Baz(Int32, , String)
end

In particular, if the splat is not the last variable of the generic type, the resulting type name is syntactically invalid. This shows up everywhere, from macro interpolations like above to Class#name and error messages.

Tuple and Proc, the only two built-in variadic generics, override Crystal::Type#to_s_with_options, so they are immune from this.