Session unable to check true / false?

I’m trying to check Session has data or not.

Empty value Session check with == false, return false.
Session with content / value check with == true, return false.
Undefined Session check with == false, return false.
Session with null as value check with == false, return false.

What is the proper way to check Session? Please advice, thank you.

P/S: I perform the test at browser console.

In the browser console:

>> Session.clear()
>> Session.keys
← Object {  }
>> Session.set('key_with_value', 123)
>> Session.keys
← Object { key_with_value: "123" }
>> Session.get('key_with_value')
← 123
>> !Session.get('key_with_value')
← false
>> Session.get('key_with_no_value')
← undefined
>> !Session.get('key_with_no_value')
← true