Fix a hang and a crash with omitted type arguments

[mame]

Found while reviewing #480.

Hang

get_instance_type and similar places created a fresh Source.new for an omitted type argument on every call. When a box typechecks against such a type, it adds an edge from that vertex to itself, so each run adds a new edge and removes the old one. Both re-queue the box, and the analysis never terminates:

class Gen[T]
  def self.create: () -> instance
end
class Object
  def accept: (Gen[Integer]) -> String
end

class Sub < Gen; end
accept(Sub.create)

This PR uses a single shared empty Source (GlobalEnv#untyped_arg) instead. An empty Source neither holds nor propagates types, so sharing it is safe.

Crash

A generic type written without arguments in RBS (e.g., -> Gen for Gen[T]) produced an Instance with no arguments. Matching it against Gen[Integer] crashed with undefined method 'each_type' for nil. The missing arguments are now filled in with their defaults or untyped.