fix: ensure session_id is set only if session is absent

Author: route

lib/ferrum/context.rb

@@ -61,8 +61,8 @@ def add_target(params:, session_id: nil)
       new_target = Target.new(@client, session_id, params)
       # `put_if_absent` returns nil if added a new value or existing if there was one already
       target = @targets.put_if_absent(new_target.id, new_target) || new_target
-      # on first iteration session_id may be null, then if session is present here we must set it to the target
-      target.session_id = session_id if session_id
+      # on first iteration session_id may be nil, then if session is present here we must set it to the target
+      target.session_id = session_id if session_id && target.session_id.nil?
       @default_target ||= target
 
       new_pending = Concurrent::IVar.new