Fix NoMethodError in SigTySingletonNode#show due to uninitialized @args

[sinsoku]

SigTySingletonNode#initialize did not initialize @Args, causing
a NoMethodError when #show was called during return type checking.
This occurred when a method annotated with singleton(X) returned
a mismatched type.

[mame]

Thank you. It seems that RBS::Types::ClassSingleton#args was added in RBS 4.0. I wonder whether we should require rbs >= 4.0.0, or branch using respond_to? instead.

[sinsoku]

Thank you for the review.

Since RBS 4.0 was recently released, I think we should maintain compatibility with RBS 3.x for now. I've updated the code to use respond_to?(:args) to avoid errors on RBS 3.x.

[mame]

Thanks!