Fix TypeError: wrong argument type NilClass (must respond to :each)

This error occurred when I tested `typeprof` on `test-unit`.
https://github.com/test-unit/test-unit/blob/24fb08fe31835b0d9ade80fb2f51554e55d1a74a/lib/test/unit/attribute.rb#L4

Although `superclass_type_args` may be nil, it was passed to `Array#zip` and raised a TypeError.
https://github.com/ruby/typeprof/blob/c440ce91d8903c9e284f911b13ca49320f6a1fb7/lib/typeprof/core/env.rb#L115

I changed it to if superclass_type_args is nil, use an empty array instead.

lib/typeprof/core/env.rb

@@ -125,7 +125,7 @@ def get_superclass_type(ty, changes, base_ty_env)
       if singleton
         Type::Singleton.new(self, super_mod)
       else
-        get_instance_type(super_mod, ty.mod.superclass_type_args, changes, base_ty_env, ty)
+        get_instance_type(super_mod, ty.mod.superclass_type_args || [], changes, base_ty_env, ty)
       end
     end
 

scenario/misc/super.rb

@@ -24,3 +24,15 @@ def foo: -> :int
 class D2 < C
   def foo: -> :str
 end
+
+## update
+class StringifyKeyHash < Hash
+  def [](key)
+    super(key.to_s)
+  end
+end
+
+## assert
+class StringifyKeyHash < Hash
+  def []: (untyped) -> untyped
+end