fix `defined?(@ivar)` with Ractors

`defined?(@ivar)` on the non main Ractor has two issues:

1. raising an exception

```ruby
class C
  @iv1 = []
  def self.defined_iv1 = defined?(@iv1)
end

Ractor.new{
  p C.defined_iv1
  #=> can not get unshareable values from instance variables of classes/modules from non-main Ractors (Ractor::IsolationError)
}.take
```

-> Do not raise an exception but return `"instance-variable"` because
it is defined.

2. returning `"instance-variable"` if there is not defined.

```
class C
  # @iv2 is not defined
  def self.defined_iv2 = defined?(@iv2)
end

Ractor.new{
  p C.defined_iv2 #=> "instance-variable"
}.take
```

-> returns `nil`

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