Correctly handle empty Paren nodes as args

[reese]

Resolves #367

Method calls like foo (arg) are represented with paren nodes by Ripper, and our deserialization previously assumed that these always had an Expression in the parens. However, you can still do things like foo (), which Ripper represents as [:paren, false] instead of something like [:paren,[:void]] or whatever.

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Note: it's worth calling out that foo () and foo are not technically the same, and that if we're being pedantic the actual output should be foo(()). Take as example the following:

def foo(arg)
  arg
end

foo () # => nil
foo # => wrong number of arguments (given 0, expected 1) (ArgumentError)

However, simply removing empty parens is the way we currently handle all equivalent cases currently, so that's the method I'm going with for this PR. I also think it's pretty fair to assume that most users would be somewhat surprised by the foo(()) transformation, and that if that's what you really mean, we can just tell users to use foo(nil) instead.